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The inexorable rise in global temperatures, combined with a strong El Niño, puts 2015 in the record books while burying a key, but bogus, argument used by those who deny that our climate is changing.

Last year is officially the hottest year in recorded history, according to independent analyses released last week by NASA and NOAA scientists. If this sounds like something you’ve heard before: It is. This time last year, those same agencies announced that 2014 had been the hottest year in recorded history. But 2014 squeaked by the previous record set in 2010 by less than five-hundredths of a degree. 2015 blew the doors off that record by 0.16°C. That may not seem like much, but it’s the largest margin by which a previous annual temperature record was broken

Globally, all but two months, January and April, were the warmest in recorded history, as the above chart shows. And this warmth wasn’t limited to some random, remote area in the middle of the ocean or Siberia. In the United States, every single state east of the Mississippi River just had its warmest December on record.

RIP “Hiatus”

Temperatures last year are the latest nails in the coffin for an unfortunately popular misconception about climate change. Called the “hiatus” or “pause,” this oft-cited talking point says that global temperatures peaked in 1998 and have not risen since.

Temperatures last year were, in fact, unusually warm in 1998, thanks in part to a strong El Niño that year. During El Niños, warmer-than-usual ocean water covers the equatorial Pacific Ocean, typically resulting in warmer-than-average global temperatures. 2015 saw the start of the strongest El Niño since 1998, and so we would expect warm temperatures in 2015, all else being equal. But, as the plot in the accompanying chart shows, all else is not equal. El Niños have been getting warmer over the past 30 years, with the 2015 El Niño just the latest, and warmest, example.

The “hiatus” is based on comparing the strong 1998 El Niño to following years, which were at best weak El Niño. 2015 was a strong El Niño year, as well, making a comparison to 1998 more reasonable. Still, any comparison of a single year to another year is not only pretty silly, it is scientifically and statistically wrong. Earth’s climate is a complicated beast, and many different factors—a lot of which are still poorly understood—cause year-to-year swings in global temperature. If we look at the records over longer time periods, though, the trend becomes obvious: Global temperatures are increasing. This would be true regardless of whether 2015 was the warmest year on record or not.

2015 Context

What does a 0.9°C warmer world actually mean? To get context, we can step back to the last time when scientists know global temperatures were quite different from today: The peak of the last ice age, around 20,000 years ago. Even though thermometers were only invented 300 years ago, scientists can infer temperatures at the last ice age by measuring biological and geochemical artifacts that are related to temperature, such as tree rings, coral growth and ice cores. A recent compilation of hundreds of these measurements by the Intergovernmental Panel on Climate Change suggest the ice age world was, on average, 4.4 ± 1.3°C colder than the 20^th century.

In other words, the temperature difference from average last year (+0.9°C) was up to 30% as large as the temperature difference at the peak of the last ice age, when ice sheets up to a mile thick covered North America down to New York, global sea level was about 450 feet lower—equivalent to the height of Great Pyramid of Giza—and wooly mammoths roamed across the high latitudes.

This isn’t an entirely fair comparison; there were undoubtedly warm and cold years during the last ice age. But the bigger point is this: Earth’s history is marked by massive physical and biological changes associated with relatively small changes in global temperature. A year that was 0.9°C warmer than average, on top of now 39 straight warmer than average years, is a change that simply cannot be ignored. Fortunately, the recent climate agreement in Paris is a key first step to potentially limit the consequences of climate change in the future, when the record warmth of 2015 will be easily forgotten under the weight of increasingly extreme years.

To Learn More

• NOAA’s 2015 State of the Climate Report: Global Analysis
• NOAA’s 2015 State of the Climate Report: U.S. Analysis

Left: Antarctica actually contains two ice sheets: the East and West Antarctic Ice Sheets. The studies released last month concern the glaciers that flow into the Amundsen Sea, the small embayment below the Antarctic Peninsula in West Antarctica. Illustration: NASA. Right: The six glaciers in the first study are shown on a close-up map of the Amundsen Sea sector. The second study focuses on one of them, the Thwaites Glacier, which drains a large area of West Antarctic Ice Sheet. Illustration: Eric Rignot

Two scientific studies on melting of the West Antarctic Ice Sheet were published recently, driving a firestorm of sensationalist headlines in the New York Times, National Geographic, CNN and other websites. Dire warnings of unstoppable glacier melt and runaway sea level rise abound. Is this all media hype? If you read no further, the answer is no — this is real.

What’s Going On?

Three major ice sheets remain on Earth today: Greenland, West Antarctica and East Antarctica. Of these, the West Antarctic Ice Sheet, though smallest by volume, is considered the most vulnerable to future melting and collapse. This is because the outer margin of the West Antarctic Ice Sheet sits on bedrock that lies below sea level, unlike the other major ice sheets. Such a “marine-based” ice sheet is sensitive to melting both on the top of the ice, from above-freezing atmospheric temperatures, and below the ice, from incursion of high-salinity ocean water.

Today, the flow of warm (relative to ice), salty ocean water beneath the West Antarctic Ice Sheet is constrained by the contact between the ice sheet and the ground beneath the ice. This is referred to as the “grounding line.” The position of the grounding line relative to the topography beneath the ice sheet is critical to determining the stability of marine-based ice sheets in response to warming.

Specifically, if the grounding line is located on a topographic upslope or ridge, the ice sheet is stable after initial retreat, because thinning and retreat of the ice sheet in response to warming is compensated by a shallower grounding line depth. If the grounding line is located behind a ridge or on a downslope, there is no topographic “catch” for the grounding line, so the ice sheet is unstable and will quickly melt and retreat.

The studies found that the grounding lines of the glaciers in the Amundsen Sea sector of West Antarctic Ice Sheet have rapidly retreated. Melting from below causes the ice sheet to thin until it is light enough to float. Illustration: Bethan Davies/AntarcticGlaciers.org

Why does this matter? Glaciers from the Amundsen Sea in West Antarctica contain enough ice to raise sea level by four feet, and have rapidly melted over the past two decades. The two scientific studies published recently provide complementary approaches to addressing the susceptibility of these glaciers to future melting. Eric Rignot, an Earth science professor at the University of California-Irvine and lead author of one study, and his colleagues provide an updated analysis of the topography of the bedrock underneath glaciers in the Amundsen Sea. The authors find that rapid retreat of these glaciers observed over the past 20 years is closely tied with elevation of the bedrock underneath the ice. Importantly, improved imaging of the bedrock underneath the current glaciers shows that, inward of the ridge upon which the grounding lines are presently located, there are no major topographic ridges or other features that may compensate for a thinning ice sheet. If these glaciers continue to melt as they are today, they will eventually enter a highly unstable regime where the entire glacier is susceptible to collapse.

Ian Joughin, who studies the physics of glaciers at the University of Washington in Seattle, and colleagues use a numerical model to forecast the timing over which such a collapse will occur. Their model correctly reproduces observed rates of melt and glacial retreat over the past 20 years, which gives confidence that the model accurately describes the timing and magnitude of melting in this glacier system. Projecting forward, the authors find that the Amundsen Sea glaciers reach a scenario of complete collapse within 200 to 500 years, and indeed are likely already experiencing an early stage of collapse today.

“Hysteresis”

Many phenomena in the Earth’s climate system are subject to hysteresis –– the dependency of an outcome on not just the current state, but on previous states as well. Hysteresis is perhaps easiest explained visually, as a variable that does not respond in a linear fashion to changes in a causal variable.

Hypothetical example illustrating the difference between linear responses and hysteretic responses. Note: this represents a major oversimplification of the climate system, and the exact relationships between CO2, temperature and sea-level rise shown here should not be accepted as scientific fact.

In the hypothetical example shown at right, the forcing agent (CO2, orange) causes a linear response in temperature (red). When CO2 rises or falls, temperature follows with a corresponding rise or fall. However, the bottom graph of sea-level rise (blue), which is assumed to be forced by temperature, shows a response that is very different from its forcing. The sea-level rise response has inertia from the previous state (increase in temperature), and so is unresponsive to the change in forcing (decrease temperature). In this hypothetical example, sea-level rise exhibits hysteresis. Even if a future corrective action was applied to ameliorate CO2 levels and force a reduction temperature, the sea-level rise persists.

The example is illustrative of perhaps the scariest aspect of Amundsen Sea glacier melting: its hysteresis. Without shallower bedrock to “catch” the grounding line, there is no known mechanism by which collapse of a marine-based ice sheet can be prevented, or reversed. Although glaciology’s knowledge of ice sheet-bedrock interactions is imperfect, the Rignot and Joughlin studies suggest complete melting of the Amundsen Sea glaciers could happen regardless of any future events.

What Happens Once Glaciers Collapse? Take Miami, Florida as an example. Miami-Dade County has a population of 2.5 million, yet averages a mere six feet above sea level today. With a four-foot sea level rise from melting of the Amundsen Sea glaciers, 40 percent of urban Miami-Dade County will be underwater. Moreover, under such a sea-level rise it would take only a two-foot storm surge to inundate 60 percent of modern-day Miami. A recent study estimated that the Miami region receives a two-foot or larger storm surge every ten to twenty years due to hurricane activity, meaning the majority of modern-day Miami would be flooded on a semi-regular basis due to natural storm activity following collapse of the Amundsen Sea glaciers.

Sea-level effects from Amundsen Sea glacier collapse would extend far beyond South Florida. Globally, an estimated 634 million people, almost 10 percent of the global population, live within 30 feet of sea-level. Although more detailed demographic-elevation estimates are not yet available, a significant fraction of this population would likely be endangered from a four-foot sea-level rise, particularly in heavily populated, low-lying areas of Southeast Asia.

Can This Be Stopped?

The West Antarctic Ice Sheet at 3 a.m. on a December day. Photo: Rutgers Institute of Marine and Coastal Science

With such risks from sea-level rise, halting the melting of the Amundsen Sea glaciers would appear to be of paramount importance. Any approach to engineer a cessation of melting, however, is quickly overwhelmed by the sheer scale of the problem. Melting of the Amundsen Sea glaciers today is driven by incursion of warmer ocean water; ceasing the melting would thus literally require cooling down the ocean.

Alternatively, the melting of the Amundsen Sea glaciers could be counteracted by an increase in ice gain from additional snowfall on the top of the glacier. However, the volume necessary is staggering: 18.6 billion Goodyear Blimps full of water would have to be frozen onto the Amundsen Sea glaciers each year, just to counteract the ~100 gigatons of ice lost to melt each year observed at present. As the ice sheet continues to destabilize in the future, this volume is expected to rise significantly.

If there is any good news, it is that the rate of collapse of the Amundsen Sea glaciers is estimated to be on the order of several centuries. In his blog, Andy Revkin points out the stark difference between geologic and societal meanings of “collapse;” for geology, a “sudden collapse” still requires multiple human generations. So we have plenty of time to prepare.

In addition, science is never settled. If the incursion of warm oceanic waters in the Amundsen Sea slows down, perhaps the glaciers will recover and avoid collapse. Of course, the opposite — increased warming and an even more rapid collapse — is equally plausible. We may yet learn of new mechanisms that lead to stabilization of marine-based glaciers. But with the state of scientific knowledge at this moment, there is little reason to suspect we can avoid, at a minimum, four feet of sea-level rise in the coming several centuries. Take your talents to South Beach soon.

Learn More

• NASA Press Release on Amundsen Sea Glacier Melting
• NASA Primer on West Antarctic Ice Sheet
• Science Journal News Release

Last of two parts

NEW YORK – Ask scientists who study ancient climates what worries them the most about a higher CO2 world and the likely answer will involve sea-level rise.

The levels of the world’s oceans and seas are ephemeral things. They rise and fall with the tides once or twice a day depending on location. Take a step back from these daily variations, though, and the “level” of the sea is anything but constant.

From the perspective of ancient climates, changes in sea level can be thought of as the addition or subtraction of water. The world’s oceans are really big bathtubs: Add water to them and their levels rise, remove water and they fall. On geologic timescales, the primary way to change the amount of water in the oceans is through the growth and decay of large ice sheets on land. Icebergs floating in the water don’t count. Like ice cubes, they merely displace water. Their melting doesn’t add any more water to the world’s bathtubs, just as a melting ice cube doesn’t cause the water in the glass to overflow.

Rob DeConto

At the peak of the last ice age about 20,000 years ago, global sea level was nearly 400 feet lower, with large ice sheets on North America and Europe accounting for much of the water removed from the ocean.

Today, most of those ice sheets are long gone, but they didn’t just gradually disappear since the last ice age. High rates of sea-level rise are particularly evident during warming intervals in the past, when atmospheric CO2 levels rose rapidly and the ice sheets melted. “We know from the geologic record that sea level has changed at rates of meters per century (during these times),” says Maureen Raymo, research professor at the Lamont-Doherty Earth Observatory of Columbia University.

Present-day rates of sea-level rise, about an inch per decade, are tiny in comparison to those in the geologic record. “In the last century, thermal expansion has been the primary driver of sea-level change,” notes Rob DeConto, professor of climatology at University of Massachusetts-Amherst. Liquids expand when heated, even a mass of water like an ocean, and this warming of the oceans has dominated recent sea-level rise.

Every indication is that thermal expansion will not dominate rates of sea-level rise in the future, however. As Earth’s climate marches toward equilibration with present-day CO2 levels, the climate will continue to warm. And this warming threatens the stability of a potentially much, much larger source for sea-level rise — the world’s remaining ice sheets.

“In this century, melting glaciers will dominate sea-level rise,” adds DeConto. “We are talking about an ability to change how the planet looks from space.”

Earth’s Ice Sheets and Sea Level Contributions: Left: The Greenland Ice Sheet (GIS) is the Northern Hemisphere’s largest. Right: Antarctica has two ice sheets, the West Antarctic Ice Sheet (WAIS), and the East Antarctic Ice Sheet (EAIS). In red is the sea level rise that would result from complete melting of ice sheet. Images from Google Earth.

Present Day Ice Sheets

To understand future sea-level change, you need to understand the ice sheets — both in terms of how much they could raise sea level, and how susceptible they are to melting with future warming.

There are three main ice sheets on Earth today. The Greenland Ice Sheet, covering nearly all of Greenland, is the only large ice sheet in the Northern Hemisphere. Over two miles thick in places, the ice sheet has an estimated 1.8 million cubic miles of ice, or enough to raise sea level by 24 feet if all the ice melted, according to research published in 2001.

In the Southern Hemisphere, Antarctica is split between two ice sheets. The smaller West Antarctic Ice Sheet has a similar volume of ice to Greenland and would raise sea level about 16 feet if it melted. In contrast, the East Antarctic Ice Sheet is by far the largest ice sheet in the world. Over two and a half miles thick across a broad area of Antarctica, the East Antarctic Ice Sheet contains nearly 14 million cubic miles of ice, and if melted, would raise sea level an almost incomprehensible 170 feet, according to 2001 research. For context, Jockey’s Ridge near Nags Head is 100 feet high.

Ice Sheet Stability

How much warming can the ice sheets withstand before they collapse? It is the million-dollar question for Earth scientists, and an area of intensive research today. DeConto and colleagues have pioneered approaches using computer models to estimate how sensitive ice sheets are to changes in temperature and CO2. Using paleoclimate information, they demonstrated that the growth of ice sheets is highly sensitive to atmospheric CO2 levels.

Maureen Raymo

Peter deMenocal

Ice sheet loss is more complicated than ice sheet growth, however. For one, the sensitivity of the ice sheet to melting depends on whether the end of the ice sheet is located on land (“land-based”) or in the ocean (“marine-based”). An ice cube in a glass of water melts much faster than one put on the counter; accordingly, marine-based ice sheets that terminate in the water exhibit faster melting rates than those that end on land.

A second complication is that ice sheets can have “multiple equilibria,” according to DeConto. From his modeling work, DeConto concludes “the Greenland Ice Sheet is relatively stable, even at today’s CO2 levels. But if you removed the Greenland Ice Sheet, it could not grow back.”

The West Antarctic Ice Sheet is the only current ice sheet that is largely marine-based, and is DeConto’s main concern for sea-level rise in a 400 ppm world. “The West Antarctic Ice Sheet is bathed in ocean water. Once the ocean gets too warm to support these ice shelves, most of the ice sheet will be lost. And sea level will then rise 3.5 meters (11 feet),” says DeConto.

Even though the melting of the West Antarctic Ice Sheet would likely take at least several hundred years, stopping the melting once it started would be nearly impossible. With warmer ocean water driving the melting, “you would have to cool the oceans down in order to get the West Antarctic Ice Sheet back (and prevent sea-level rise),” adds DeConto. “That’s essentially an impossible geoengineering problem.”

Although the Greenland and East Antarctic Ice sheets are not considered risks for catastrophic melting at current CO2 levels, some degree of melting from both would also contribute to sea-level rise. As a result, “If you stabilize CO2 at current levels, a five-meter (16-foot) rise in sea level is not out of the realm of possibility,” cautions Raymo. “Think about what that would do to the world’s population centers”. An interactive feature published last fall by The New York Times gives several examples of how sea-level rise could affect coastal urban areas in the United States.

DeConto is quick to caution that modeling studies of ice sheet growth and decay have plenty of limitations. Alarmism is, unfortunately, not one of them. “Time and again, we have a problem making models as sensitive as they need to be. They are not sensitive enough to match the geologic record. If anything, our models are too conservative. And that’s scary,” says DeConto.

Changing Cities with 12 Feet of Sea-level Rise: Twelve feet of sea-level rise would occur with melting of the marine-based portions of the West Antarctic Ice Sheet. Images:  Baden Copeland/New York Times.

Only Getting Slushier

Unfortunately, 400 ppm CO2 is not this story’s end. According to NOAA, atmospheric CO2 levels were increasing by 2.65 ppm per year in 2012, the highest year-over-year increase since recordkeeping began in 1959. If these trends continue, “the West Antarctic Ice Sheet will deglaciate, it’s only a question of time,” says DeConto.

At current trajectories, atmospheric CO2 levels could approach 800 to 900 ppm by the end of the century. “That plunges you into a time of warmth from 50 million years ago that you wouldn’t even recognize,” says Raymo. “All the ice sheets would eventually melt over the next few thousand years. Sea level would end up being over 60 meters higher.”

Were that to happen, everywhere east of I-95 in North Carolina would eventually be underwater. Fayetteville would become beachfront property.

The economic costs associated with these scenarios are difficult to contemplate. As it is, “We’re starting to see the economic costs of climate change now, especially in insurance,” contends Peter deMenocal, professor and chair of the Department of Earth and Environmental Sciences at Columbia.

“Consider federal home insurance for coastal cities. At what point of sea-level rise will that become unaffordable for the federal government?” says deMenocal. “We’re talking about trillions of dollars here.”

And the major players in global sea level—the ice sheets — have yet to join the game.

First of two parts

NEW YORK — The National Oceanic and Atmospheric Administration announced on May 10 that concentrations of atmospheric carbon dioxide, or CO2, surpassed 400 parts per million, or ppm, in the clean air above Mauna Loa, Hawaii, where researchers have been testing the air for CO₂ for more than 50 years.

Ordinarily, a gas in the atmosphere passing a certain level would attract little attention, even from scientists. CO2 is a different story, but why? Why should society care that CO2 is now as high as 400 ppm? The reasons are multiple, but all trace back to the relationship between CO₂ and temperature.

Today, we’ll learn why scientists place so much emphasis on the amount of CO2, and how today’s levels compared to the past. Tomorrow, we‘ll explore the effects of a 400 ppm CO2 world, particularly how it might affect sea-level rise.

Ice core records of CO₂ and climate over the past million years: The concentration of CO₂ in air bubbles within Antarctic ice cores (orange, scale at left) and change in Antarctic temperature relative to today (blue, scale at right). Present-day CO₂ concentration is indicated by the orange dashed line. The chart shows that temperature and CO₂ are tightly coupled over time, with higher CO₂ corresponding to warmer temperatures. Data from NOAA Paleoclimatology Program

Why is CO2 important for temperature?

First and foremost, CO2 is no ordinary gas. Though a minor constituent of Earth’s atmosphere, CO2 plays an outsized role in influencing climate because of its “greenhouse” properties.

Barbel Honisch

The structure of a carbon dioxide atom allows it to reflect radiation emitted by the Earth’s surface that would otherwise return to space, trapping heat like the glass panes of a greenhouse. This can get very complicated very quickly. The web site Skeptical Science has an in-depth examination of the subject for those wanting more detail.

But here’s the bottom line: When atmospheric CO2 concentrations increase, more outgoing radiation is absorbed, leading to warming of the Earth surface.A byproduct of burning fossil fuels, the concentration of carbon dioxide in the upper atmosphere has been steadily increasing since the dawn of the Industrial Age.

The relationship between CO2 and climate is especially evident when looking back through Earth’s history. A particular field of environmental science, paleoclimatology, studies past climates using clues from archives of Earth’s history, including rocks, sediments and ice. With information gathered from these archives, scientists seek to understand how and why Earth’s climate has changed in the past and what these mechanisms mean for current and future climate change.

“Paleoclimate studies provide very strong evidence that there is a tight connection between CO2 and climate,” notes Bärbel Hönisch, an assistant professor in the Department of Earth and Environmental Sciences at Columbia University. Gas bubbles found in ice cores from Antarctica over the last 800,000 years demonstrate the close linkage between past atmospheric CO2 levels and past temperatures, estimated from the ice’s composition. Taken in the context of that long-term record, atmospheric CO2 today is nearly 50 percent higher than the highest level measured in the ice cores.

400 in Context: The Pliocene

For paleoclimatologists, the 400 ppm CO2 level is significant not so much for the absolute number, but rather because it suggests a return to a type of climate not seen on Earth for millions of years. “We’re passing through a number easily divisible by 100, but a year from now it will be 402 [ppm]… it’s an inexorable climb to higher values,” says Peter deMenocal, professor and chair of the Department of Earth and Environmental Sciences at Columbia University.

As a consequence of this climb, “We are starting to head back into deep geologic time,” notes Rob DeConto, professor of climatology at University of Massachusetts-Amherst.

Rob DeConto

Maureen Raymo

When was the last time CO2 levels were 400 ppm? Scientists know from ice cores that this cannot be any point in the past million years or so. To look at time periods before the ice cores, CO2 cannot be measured directly; rather, it has to be inferred from the geologic record. By measuring “proxies” of atmospheric CO2, such as tree rings, coral growth and other physical or chemical variables that relate to atmospheric CO2, scientists can reconstruct CO2 levels in the past with a reasonable degree of precision.

But such measurements are not smooth sailing. An expert in CO2 proxies, Hönisch is candid about the difficulties. “There are only a handful of studies to reconstruct past CO2. It’s difficult—a lot of these proxies are in their infancy, and they are challenging and expensive measurements to make,” she says.

Nonetheless, the records produced by Hönisch and other laboratory groups converge on showing that the last time CO2 levels were around 400 ppm was some 3 to 5 million years ago during what is known by geologists as the Pliocene Epoch. “We’re definitely in the Pliocene now,” agrees Maureen Raymo, research professor at Columbia’s Lamont-Doherty Earth Observatory.

That the Pliocene is an analog for today’s CO2 levels is little comfort to paleoclimatologists. “If you go back to the Earth 3 million years ago, it was a very, very different world than today,” says deMenocal.

For instance, DeConto, Hönisch and Raymo all note that Greenland covered in nearly a two mile-thick ice sheet today, was largely ice-free in the Pliocene. Scientists estimate that global sea level during the Pliocene was anywhere between 30 and 90 feet higher than it is today, according to a 2012 study. Temperatures along the East Coast of the United States were likely 5 degrees warmer on average than today. That may not sound like much, but imagine every 95-degree summer day being 100 degrees instead.

At higher latitudes, the temperature changes were even greater. Ellesmere Island, high within the Arctic Circle, is a striking example of the difference between the Pliocene and the present. Today, Ellesmere Island is covered with snow and glaciers, only reaching temperatures above freezing during the height of summer. But back in the Pliocene, Ellesmere Island was covered with forests, and was even home to an ancestor to modern camels. Evidence from these flora and fauna suggest Ellesmere Island was, on average, a whopping 32 degrees warmer than today during the Pliocene.

Ellesmere Island, then and now: Left: Artist’s rendition of High Arctic camels on Ellesmere Island during the Pliocene. Illustration:Julius Csotonyi/Canadian Museum of Nature. Right: The Osborn Range on Ellesmere Island at present day. Photo: Ansgar Walk,Wikimedia Commons.

Out of Equilibrium

Now that CO2 has hit 400 ppm, why haven’t camels returned to Ellesmere Island? And why didn’t the temperature on Ellesmere increase by 32 degrees on May 11?

Peter deMenocal

The reason is that climatic response to CO2 is not instantaneous. “Our climate is vastly out of equilibrium with CO2 levels today, largely because of the oceans,” says deMenocal. Water has a large capacity to accumulate excess heat. Add up all the water in the ocean and you get a massive heat sink that takes a long time to adjust to the impact from CO2 changes.

Because they encompass broader time intervals, paleoclimate records give an image of what conditions look like at equilibrium. “The Pliocene is a good analog for how future conditions will be when they come to equilibrium with the current concentration of CO2, but that equilibrium can take two to three thousand years,” says Raymo. In short, we haven’t reached the point yet where the world’s climate behaves as would be expected in a 400 ppm world.

But we are well on our way. Even though it takes millennia to achieve equilibrium between CO2 and temperature, some effects of higher CO2 are already being realized. Parts of the climate system that are highly sensitive to changing CO2, such as Arctic sea ice and polar temperatures, are already responding. And the world is warming. “If you were born in the 1990s, almost every year of your life has been a record warm year. It’s remarkable,” says deMenocal.

“We are living in a world where the rate of change is accelerating,” adds deMenocal. “It’s like steering a supertanker through a narrow channel, but we have no idea how to steer the ship. And the ship is gaining speed.”

Thursday: The seas in a high CO2 world

Tuckerton, N.J., seems adrift at sea after Sandy passed on Oct. 30. Can residents of the Jersey shore expect more of the same in the future? Photo: Coast Guard.

NEW YORK — In the aftermath of a storm that left more than 100 dead, over 8.5 million without power and caused an estimated $50 billion in damage, scientific experts and coastal residents alike are grappling with the portent of Superstorm Sandy.

Was this an incredibly damaging but incredibly rare strike on New York and New Jersey? Or is Sandy a harbinger of things to come, of a climate system driven to extremes by manmade greenhouse gas emissions?

For scientists convinced that global climate will undergo dramatic changes in the coming decades, extreme events like Sandy serve as something of a flashpoint. On the one hand, there is always a probability, however low, that an extreme event will occur within a given year. On the other, a warming climate fundamentally alters the probability of such extreme events.

Sandy’s Track and Surge

The track of Hurricane Sandy (red line), Hurricane Irene (orange line), and all tropical storms and hurricanes from the NOAA Hurdat2 historical database that crossed within 3° latitude and 5° longitude of Long Island, New York (blue lines).

Conflating the issue is the very unusual track of Sandy. Forming as a depression in the Caribbean Sea on Oct. 22, Sandy strengthened into a late-season hurricane while heading northward through Jamaica, Cuba and the Bahamas. Off the coast of the Carolinas, Sandy interacted with a low-pressure system over the eastern United States, transitioning into a large and powerful hybrid storm before taking a westward turn into southern New Jersey on Oct. 30. Historically, most storms in this region, including Irene in 2011, have tracked to the northeast.

“There is no storm like it in the historical database,” noted Adam Sobel, professor of applied physics and applied mathematics at Columbia University and an expert on tropical meteorology. “The track was a worst-case scenario for New York City.”

Strikes by tropical systems on New England are, by themselves, not usual: tropical storm or hurricane landfalls along the coast between New Jersey and Massachusetts occur about every six to 10 years, according to a study published in 2007 in the Journal of Climate. But the storm surge from Sandy in New York City was likely larger than any storm in recorded history. A study published this year by Ning Lin, Kerry Emanuel and colleagues in Nature Climate Change suggests that the storm surge from Sandy was a once in 700 years event .

The Attribution Paradox

Adam Sobel

Kerry Emanuel

Joerg Schaefer

Any search for a link between Superstorm Sandy and global climate change is hampered by a fundamental issue: Sandy was just one storm. And in the realm of noisy weather patterns, practically anything is possible given enough time, even without climate change.

“There are two logical possibilities,” said Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology. “One is that we were extraordinarily unlucky, and Sandy was a very rare event that we just happened to see this year. The other is that some change in the system has made events like this more probable.

“The big problem is that Sandy was a hybrid event (a combination of a hurricane and a nor’easter), and we haven’t done our homework on hybrid events,” Emanuel continued. “It’s impossible to know without more work.”

On the link between Sandy and climate change, Sobel is “skeptical but not dismissive”.

“It’s very hard to connect one event to climate,” he said. “But we’ve had two storms [Irene and Sandy] in two years and this track that’s never happened before in 150 years. We have to have an open mind when trying to determine the links between climate and severe weather events, even if we don’t yet understand all of them.”

For Joerg Schaefer, research professor at the Lamont-Doherty Earth Observatory, proving the influence of climate change on Sandy is “putting the burden of proof on the wrong shoulders”.

“Everything behaves as we’d predict in a warmer world,” Schaefer said, citing the record loss of Arctic sea ice this summer as the latest in a litany of global weather extremes over the last decade. “The question needs to be turned around: Can we prove that climate change did not affect Sandy?”

As longtime residents of coastal areas can attest, hurricane-prone regions have witnessed dramatic increases in population, population density and development over the past decades. All have led to an intensification of the damage incurred from storms, even if the storms are not getting stronger. But add stronger storms on top of this, and the losses mount precariously.

“Over the last 100 years, the vast majority of the increase in hurricane damage is from demographics, not climate”, noted Emanuel, who studies hurricane losses and climate change. “But the two are multiplicative, not additive.”

“Even 150 years ago before CO₂ was rising, the Jersey Shore, Battery Park and the Rockaways were risky places to develop,” said Schaefer. “But what climate change does is enhance the risk.”

Future Trajectories

Going forward, Emanuel, Schaefer and Sobel agree that extreme events may become more likely due to anthropogenic climate change. The modeling work by Lin, Emanuel and colleagues suggests that the probability of a Sandy-sized storm surge event, a once-in-700 years event today, could increase to better than a once-in-300 years event by 2100, owing to sea-level rise and an increase in the strength of hurricanes from climate change.

“The right question to ask is, ‘Do we have a good reason to expect that the probability of an event like Sandy will increase in a warmer climate?’ CO₂ is pushing us in one direction, on top of what the variability is, which may make extreme events like this more probable,” said Sobel.

“There should be a presumption of some unknown degree of climate influence on unusual weather by this point,” continued Emanuel. “It’s a question of how we should treat risk.”

If there is risk for more frequent or stronger hurricanes, then there is at least one silver lining in Sandy: The hurricane forecasts are better than they have ever been, thanks in no small part to decades of government funding for basic and applied research. “In Katrina, the forecast was very good and in Sandy, the forecast was uncannily good,” noted Sobel. “These were directly the result of long-term, government-financed improvements in forecasts and models”.

In the meantime, the question on everyone’s mind is obvious: When will the next Sandy strike? “I don’t know whether we have any better ability to predict that now than we did a year ago,” said Sobel.

“I don’t want to make predictions, but we’d be smart to start preparing,” he said.

Figure 1: July 2012 Temperature anomalies Source: NOAA

If you thought it was hot in July in coastal North Carolina, you’re not alone. The month was the hottest month in recorded history for the United States. Following the exceptionally warm winter and early spring, we may be getting an early glimpse of how future climate will look.

On August 6, NOAA’s National Climatic Data Center reported that July was the single warmest month in the 117-year temperature history of the continental United States. During the month, over half the continental United States experienced temperatures more than 2 degrees above average, an unprecedented area and magnitude of warmth (Figure 1).

July’s record warmth was just the latest in an exceptional year of heat for the continental United States. A late June super-heat wave led to 208 locations tying or breaking their all-time highest temperature records, despite that June isn’t even the warmest month of the year. From Alaska to Arkansas, a vast swath of the country set or tied their highest-ever June temperatures during this heat wave (Figure 2).

And the hot summer is a continuation of the status quo for past several months. March was the warmest March in recorded history for the continental United States. So far, this year is more than 4 degrees warmer than the 20^th century average, smashing records of previous warm years (Figure 3).

All told, the past 12 months have been the warmest 12-month period in recorded history for the Unites States. In fact, since May of 2011, every month has been warmer than its climatological average, a 14-month stretch of warmer-than-average temperatures. What are the odds that 14 straight months would be warmer than average?

Of course, 14 straight months of anomalous warmth are more likely if the chance of warmth is better than 50/50. Since the 1980s, scientists have postulated that carbon dioxide-forced global warming would lead to global increases in the frequency of heat waves and droughts, in effect “stacking the odds” in favor of warmer conditions.One can assume there is a 50 percent chance that any given month will be warmer (or cooler) than average. The odds of having 14 such warmer-than-average months in a row, as we’ve had since May 2011, would thus be equivalent to the odds of flipping a coin heads 14 times in a row: one in 16,000. It should be noted here that this probability assumes that the chances of any given months being warmer or cooler than average are independent of all the other months, which isn’t necessarily true. In reality, the odds are probably slightly higher than one in 16,000, but difficult to quantify exactly.

In 1981, climatologist Jim Hansen of the Goddard Institute of Space Studies, writing in the journal Science, hypothesized that “the creation of hot, dry conditions in much of the western two-thirds of the United States” as a result of additional atmospheric carbon dioxide.

Thirty-one years later, Hansen’s predictions appear substantiated, as the United States is currently in grips of the worst drought since the 1930s Dust Bowl. Almost two-thirds of the country is experiencing drought conditions, and the U.S. Department of Agriculture is projecting that the 2012/2013 corn yield will be the lowest in 17 years due to “extreme heat and dryness” across the Great Plains. As a consequence, global food prices spiked 6% in July, led by a 17 percent increase in the price of cereals and a 23 percent surge in the price of corn (Figure 4).

As with any climatic changes, there are winners and losers. Locally, farmers in North Carolina may gain from the high corn prices, with the USDA projecting a 30 percent increase in N.C. corn production over last summer despite the drought conditions. Globally, however, rising world food prices tend to exacerbate political and social conflict, especially in developing countries, according to the World Bank.

Will the warmth continue? As far as the record books are concerned, it doesn’t matter: the first half of 2012 has been so warm that, even if temperatures for the rest of year are in line with 20^th century averages, 2012 will still be the warmest year in U.S. history, according to the National Climatic Data Center in Asheville.

And with a developing El Niño historically favoring a warmer late fall in the central and eastern United States (Figure 5), it is likely that the second half of 2012 will continue to be warmer than average.

Globally, the hot summer of 2012 in the United States contrasts with cooler-than-average conditions in the United Kingdom and other locations. But the balance isn’t complete: Like every year since 1977, global temperatures in 2012 will be above the 20^th century average.

By 2030, global temperatures are expected to be 0.9 degrees warmer than today, rising to almost 2 degrees warmer than today by 2050. With such an increase in background warmth, hot months, and hot years, will undoubtedly become hotter.

So go ahead and pencil in 2012 as the warmest year in US history. Just don’t expect it to stay in the record books for very long.

NEW YORK – Friday marks the beginning of the 2012 Atlantic hurricane season. Cue the groans, the crossed fingers and the hope that mad rushes for plywood and batteries will wait for another year. Many of you are probably wondering what’s the chance that you will get hit this year.

As residents of Eastern North Carolina know well, hurricanes are not idle threats. According to the National Climatic Data Center, tropical storm and hurricane strikes are the single most common causes of billion-dollar natural disasters in the United States, accounting for nearly $260 billion in damages between 1980 and 2005, or more than half of the combined losses from all U.S. natural disasters. And since 1851, 18 percent of all hurricane strikes on the United States occurred in North Carolina.

Part of the frustration with hurricanes—and one reason why they are so destructive—is that hurricane strikes are anything but predictable. Along the North Carolina coast, the total number of storm that make landfall varies enormously from year to year. For instance, between 1986 and 1995, the N.C. coast was directly struck by only one hurricane (Charley in 1986) and brushed by another (Emily in 1993). However, six hurricanes would make landfall along the coast over the next 10 years (Bertha and Fran in 1996, Bonnie in 1998, Dennis and Floyd in 1999, and Isabel in 2003). Back in 1955, three hurricanes—Connie, Diane, and Ione—struck the NC coast within a six-week span.

Average number of yearly Atlantic hurricanes per decade (blue line), and number of hurricanes striking within 150 miles of North Carolina (red bars). Atlantic hurricane counts from wunderground.com/hurricane/hurrarchive.asp, North Carolina hurricane statistics from nc-climate.ncsu.edu/climate/hurricanes/statistics.php.

Such great variability in the number hurricane landfalls demands an explanation. Intuitively, the number of landfalls reflects the total number of hurricanes: the more hurricanes in a season, the greater chance that any area could receive a direct hit. For North Carolina, the number of hurricane impacts per decade tracks the average number of yearly storms, as the accompanying chart shows.

One way to estimate the chance of a hurricane strike to a certain area is to estimate the total number of storms during a hurricane season. For such estimates, it is crucial to have accurate records of past hurricane strikes. Unfortunately, historical data are limited in length, only going back to the mid-19th century, and their accuracy and coverage are questionable at best.

For Jeff Donnelly, an associate scientist at Woods Hole Oceanographic Institution in Massachusetts, extending the historical record of hurricane strikes is a matter of digging deeper. Just not into library archives or journals; instead, Donnelly searches for evidence of hurricanes in salt marshes on the landward side of barrier islands.

Over the last two decades, scientists have increasingly turned to clues from the earth for records of past hurricane strikes. This approach, termed paleotempestology (paleo- past, tempest- storms; logy- the study of), relies on a simple principle: Hurricanes tend to move things to where they normally would not be found.

In salt marshes, the material moved is sand. When a hurricane approaches the coast, wave action and storm surge erodes sand from the beach and brings it inland, depositing the sand on top of the muddy sediment in the salt marsh. After the storm surge recedes, the marsh recovers, and muddy sediment is again deposited on top of the sandy layer. The end result, according to Jon Woodruff, assistant professor at University of Massachusetts-Amherst and a former student of Donnelly, is “the perfect dirt layer cake”.

Donnelly and Woodruff take cores in the landward salt marshes, and reconstruct past hurricane strikes from the sand layers in these tubes of sediment. Previous work published by Donnelly and colleagues has recovered evidence for major hurricane strikes in the 18th, 19th and early 20th centuries in the salt marshes of New Jersey, Long Island and Rhode Island that coincided with documented hurricane strikes.

But the potential doesn’t stop there. With longer sediment cores and the right location, hurricane strikes can be inferred from times long before the historical record. “Cores help us take a look at history over 5,000 years, and that’s a powerful tool,” Donnelly told a Oceanus magazine in 2009.

A sediment sample collected from the Florida Panhandle. The dark sediment is mud that is normally deposited in a marsh. The lighter bands of sediment represent layers of sand that are washed into the marsh during hurricanes. Photo credit: Jon Woodruff

A recent hurricane strike reconstruction from Puerto Rico, published in Nature, suggests that increased hurricane activity in the North Atlantic over the last 5,000 years generally corresponds to weaker El Niño events and a stronger West African monsoon.

Despite their potential, reconstructions of past hurricane strikes have proved challenging to generate from eastern North Carolina. A 2006 study by Steven Culver and colleagues from East Carolina University examined salt marsh cores from Pea Island. The authors showed that sand deposits in these salt marshes were far too variable and widespread to be explained by hurricane activity and likely related to changes in inlet positions along Pea Island. Similar research in Onslow Bay by Scott Hippensteel of University of North Carolina-Charlotte has found evidence for storm activity in Onslow Bay over the past 1,500 years.”

Hurricanes will never cease to be a risk to the residents and economy of eastern North Carolina. But with methods like paleotempestology, scientists are gaining a more complete picture of past hurricane variability and of what factors may contribute to future hurricane variability. While scientists may never be able to predict the chance of a hurricane strike in one location in any given year with certainty, research into past hurricane strikes has certainly underscored the dynamic history of the coasts we call home.

Editor’s note:The N.C. Coastal Resources Commission at its meeting in Beaufort today will once again consider a coastal planning policy for sea-level rise. The commission’s panel of scientific advisors presented the commission with a draft policy in March 2010. It projected that sea level along the N.C. coast could rise 15 to 55 inches by 2100 because of a warming climate. That forecast and most planning recommendations were removed from a later draft after objections from development interests and some coastal counties. The commission asked the Coastal Hazards Science Panel, its advisors, to respond to the objections. The panel did so recently in an “addendum” to its original report in which the so-called “Science Panel” unanimously reaffirmed its findings. All of the authors, except for Dr. Reide Corbett, are members of the panel.

By Robert S. Young, D. Reide Corbett, David Mallinson, Charles “Pete” Peterson, Stanley R. Riggs, Antonio B. Rodriguez

We would like to take this opportunity to add our own personal perspective to the just completed “addendum” report authored by the Science Panel. It should come as no surprise that the panel unanimously voted to support the science behind the projections in our first report. The Science Panel is composed of very well qualified scientists and engineers, some of whom have spent their careers examining the nature and impacts of changing sea levels in North Carolina. The state is fortunate to have such expertise available for consultation.

Dr. Robert S. Young

Dr. Reide Corbett

Dr. David Mallinson

Dr. Charles “Pete” Peterson

Dr. Stanley Riggs

Dr. Antonio B. Rodriguez

No one wants to think about the potential for a meter of sea level rise, but this projection is clearly the national and international consensus being used for planning the potential impacts of rising sea level over the next century.

Much has been said in the media over the last few months, and we respect the right of all North Carolinians to have their say, and to participate in the discussion. We would like to address a few of what we believe are important misunderstandings or misconceptions:

There is no scientific consensus on global climate change and the expected acceleration of sea level rise:  There is, in fact, clear scientific consensus on global climate change amongst scientists. Every major scientific organization in the United States, Europe, and Asia has produced statements supporting the science behind the human contribution to global warming and indicating an expected acceleration in the rate of sea-level rise over the next century.

There are other viewpoints, but they do not represent the vast majority of scientists. We like to think of it this way. If you had a persistent cough, and you went to 100 doctors for a diagnosis and 98 doctors said you had lung cancer, while two doctors said you simply had a throat tickle, we are guessing that you would take the cancer diagnosis seriously. You might even begin to seek treatment. In the case of sea-level rise, 98 of the 100 scientists are telling you that there will be an acceleration in the rate of rise this century. It is up to you to decide what to do about it.

These scientific organizations embrace climate change so that they can continue to get grant money:  This assertion is a bit absurd. Organizations like the National Academies, the American Association for the Advancement of Science, the Geological Society of America, the American Geophysical Union and many others have all supported the science behind global change. Yet, a very, very small percentage of their members conduct funded climate change research. Rather, these diverse groups of scientists have examined the evidence and, based on the facts, found it to be convincing enough to produce scientific statements regarding the need for public action and continued research.

If it is true that these policy statements will lead to an increase in the funding for global change research, then all of the other scientists in these organizations have just “shot themselves in the foot.” There is only so much research money to go around. If it is all going to climate change, then the other scientists in these many organizations are likely to have less funding available for their particular research interest.

A far more likely scenario than a conspiracy to get funding is this: The scientists and engineers in these organizations looked at the peer-reviewed literature and concluded that global change is real, and that an expectation for an acceleration in the rate of sea-level rise is reasonable.

Even talking about one meter of sea-level rise will impact insurance rates and harm the coastal economy:  We find this hard to believe. First of all, the insurance companies are well aware of the science behind global climate change and the prevailing projections of sea-level rise. They have funded much research into the science, hazards and risks associated with sea-level rise, coastal erosion and storms. Nothing in the Science Panel’s report or the discussion at the Coastal Resources Commission will come as a surprise to the insurance industry.

Second, the real and immediate threat along the N.C. coast remains property damage resulting from storms. Some areas of the United State have experienced multiple storm impacts; yet, their coastal economies continue to thrive. Dauphin Island, Ala., has been nearly wiped off the map several times in the last 30 years. Each time the barrier island was rebuilt, and the economy there chugs along. One might question whether or not this is good policy, but it is hard to imagine that something as abstract as a 100-year projection for rising sea level is going to impact the coastal economy when the very real prospect of storm impacts has not slowed coastal growth.

The scientists and engineers who project a one-meter sea-level rise by 2100 are calling for an abandonment of the coast:  This is simply not true. Currently, the rate of sea-level rise does not appear to be accelerating (although scientists do expect that to change in the future). If you are building a single-family home, or a subdivision, you probably don’t need to do anything other than account for storms. In many respects, the best way to prepare for sea-level rise over the next two to three decades is simply to do a better job of preparing for major storm impacts. If you are building a nuclear power plant, an airbase or a major port facility (all of which are expected to last longer than a single-family home), you should take the potential for sea-level rise into consideration.

We may never get to that projected one meter of sea-level rise: We WILL experience a sea-level rise of one meter.  We don’t know if it will be 75 years from now or 150 years from now, but we will get there. As we do, the map of Eastern North Carolina will change. It has already changed dramatically from the last 500 years of sea-level rise.  The uncertainty in the future rate and acceleration of global sea-level rise is the main reason why the Science Panel suggested updating and reviewing the report’s projections every five years. Regardless, it is perfectly reasonable for local and state planners to begin consideration of how the map may change in order to develop very long-term plans for adjusting. This is done in order to preserve and protect the coastal economy, not to plan its abandonment.

One final note, we all have a stake in the preservation of North Carolina’s coastal resources, North Carolina’s coastal economy and the wise management of both. Discussing the potential for future sea-level rise in a frank, polite and constructive manner is our effort to contribute to that wise management. We hope that it will be received as such.