A significant rise in sea level is inevitable by the end of this century; ocean temperatures over the last 40 years have risen far above naturally occurring climate cycles; and continued greenhouse gas emissions may overwhelm the capacity of land and ocean to absorb excess carbon dioxide, thus speeding up global warming. Those are the conclusions of three recent climate modeling studies, all pointing to the inevitability of global warming and the need to develop mitigation strategies.

Even if the concentrations of all greenhouse gases had leveled off in the year 2000, we would still be committed to a warmer Earth and rising sea levels in this century, according to a study by a team of climate modelers at the National Center for Atmospheric Research (NCAR).1 The models were run on supercomputers at NCAR and several DOE labs, including NERSC, and on the Earth Simulator in Japan.

The modeling study quantifies the relative rates of sea level rise and global temperature increase that we are already committed to in the 21st century. Even if no more greenhouse gases were added to the atmosphere, globally averaged surface air temperatures would rise about 0.5° C (1° F) and global sea levels would rise another 11 centimeters (4.3 inches) from thermal expansion alone by 2100.

“Many people don’t realize we are committed right now to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere,” said lead author Gerald Meehl. “Even if we stabilize greenhouse gas concentrations, the climate will continue to warm, and there will be proportionately even more sea level rise. The longer we wait, the more climate change we are committed to in the future.”

The half-degree temperature rise is similar to that observed during the second half of the 20th century, but the projected sea level rise is more than twice the 5-centimeter (2-inch) rise that occurred during that period. These numbers do not take into account fresh water from melting ice sheets and glaciers, which could at least double the sea level rise caused by thermal expansion alone.

Though temperature rise shows signs of leveling off 100 years after stabilization in the study, ocean waters continue to warm and expand, causing global sea level to rise unabated (Figure 1).

“With the ongoing increase in concentrations of greenhouse gases, every day we commit to more climate change in the future,” Meehl explained. “When and how we stabilize concentrations will dictate, on the time scale of a century or so, how much more warming we will experience. But we are already committed to ongoing large sea level rise, even if concentrations of greenhouse gases could be stabilized.”

Figure 1. Percent increase of globally averaged surface air temperature and sea level rise from the two models computed relative to values for the base period 1980–1999, for the experiment in which greenhouse gas concentrations and all other atmospheric constituents were stabilized at the end of the 20th century.

The inevitability of the climate changes described in the study is the result of thermal inertia, mainly from the oceans, and the long lifetime of carbon dioxide and other greenhouse gases in the atmosphere. Thermal inertia refers to the process by which water heats and cools more slowly than air because it is denser than air.

This study quantifies future committed climate change using coupled global three-dimensional climate models. Coupled models link major components of Earth’s climate in ways that allow them to interact with each other. Meehl and his NCAR colleagues ran the same scenario a number of times and averaged the results to create ensemble simulations from each of two global climate models. Then they compared the results from each model.

The scientists also used the two models to compare possible 21st century climate scenarios in which greenhouse gases continue to build in the atmosphere at low, moderate, or high rates. The worst-case scenario projects an average temperature rise of 3.5° C (6.3° F) and sea level rise from thermal expansion of 30 centimeters (12 inches) by 2100 (Figure 2). All scenarios analyzed in the study will be assessed by international teams of scientists for the next report by the Intergovernmental Panel on Climate Change, due out in 2007.

The NCAR team used the Parallel Climate Model (PCM), developed by NCAR and the Department of Energy, and the new Community Climate System Model (Version 3). The CCSM3 was developed at NCAR with input from university and federal climate scientists around the country and principal funding from the National Science Foundation (NCAR’s primary sponsor) and the Department of Energy. The CCSM3 shows slightly higher temperature rise and sea level rise from thermal expansion and greater weakening of the thermohaline circulation in the North Atlantic. Otherwise, the results from the two models are similar. Three of the researchers later performed the same analysis using 12 different general climate models, and all showed similar results.

Figure 2. Globally averaged sea level rise from thermal expansion for various scenarios of greenhouse gas buildup.

The warming of the oceans over the past 40 years has not been uniform, and another recent study2 investigated warming at different depths on an ocean-by-ocean basis, using observational data as well as simulations from PCM and the HadCM3 model from Britain’s Hadley Centre. The researchers noted that the vertical structure of the temperature data varies widely by ocean, and they explored three possible causes for the warming trend: (1) natural variability internal to the coupled ocean–atmosphere system; (2) external natural variability, such as solar or volcanic forcing; and (3) forcing arising from human activity (emission of greenhouse gases and sulfate aerosols).

The study showed that the warming trend is much stronger than would be expected from natural variability. In Figure 3, the hatched blue region shows simulated natural internal variability; the green triangles represent combined solar and volcanic forcing; and the red dots represent observed temperature trends. On the time and space scales used in this study, the solar plus volcanic signals are generally indistinguishable from the natural internal variability, but the observed temperatures bear little resemblance to the natural variations. Figure 4 shows that the actual temperature trends do match the simulated results of greenhouse gases and sulfate aerosols, indicating that ocean warming is caused by human activities.

“This is perhaps the most compelling evidence yet that global warming is happening right now and it shows that we can successfully simulate its past and likely future evolution,” said Tim Barnett, lead author of the study and a research marine physicist at the Scripps Institution of Oceanography. Barnett said he was “stunned” by the results because the computer models reproduced the penetration of the warming signal in all the oceans. “The statistical significance of these results is far too strong to be merely dismissed and should wipe out much of the uncertainty about the reality of global warming.”

Figure 3. Observed ocean temperature trends from the last 40 years (red dots), projected onto a PCM simulation of natural variability internal to the coupled ocean–atmosphere system (blue hatched area) and combined solar and volcanic forcing (green triangles). Actual temperatures are much higher than would be expected from natural variability.

Figure 4. Observed ocean temperature trends (red dots) projected onto a PCM simulation of the results of greenhouse gases and sulfate aerosols (green hatched area). There is excellent agreement at most depths in all oceans, indicating that ocean warming is the result of human activities. Similar results were obtained with the HadCM3 model.

A third study, based on a climate model that includes the effects of Earth’s carbon cycle, indicates there are limits to our planet’s ability to absorb increased emissions of carbon dioxide.3 If the current release of carbon from fossil fuels continues unabated, by the end of the century the land and oceans will be less able to take up carbon than they are today, the model indicates.

“If we maintain our current course of fossil fuel emissions or accelerate our emissions, the land and oceans will not be able to slow the rise of carbon dioxide in the atmosphere the way they’re doing now,” said Inez Y. Fung at the University of California, Berkeley, who is director of the Berkeley Atmospheric Sciences Center, co-director of the new Berkeley Institute of the Environment, and professor of earth and planetary science and of environmental science, policy and management. “It’s all about rates. If the rate of fossil fuel emissions is too high, the carbon storage capacity of the land and oceans decreases and climate warming accelerates.”

Currently, the land and oceans absorb about half of the carbon dioxide produced by human activity, most of it resulting from the burning of fossil fuels, Fung said. Some scientists have suggested that the land and oceans will continue to absorb more and more CO₂ as fossil fuel emissions increase, making plants flourish and the oceans bloom. Fung’s computer model, however, indicates that the “breathing biosphere” can absorb carbon only so fast. Beyond a certain point, the planet will not be able to keep up with carbon dioxide emissions (Figure 5).

Figure 5. The impact of carbon–climate feedback on carbon storage for the years 2001–2100 (a) in the oceans and (b) on land. The yellow, green, and blue areas indicates less carbon storage, while the orange and red areas indicate more carbon storage.

“The reason is very simple,” Fung said. “Plants are happy growing at a certain rate, and though they can accelerate to a certain extent with more CO₂, the rate is limited by metabolic reactions in the plant, by water and nutrient availability, et cetera.”

Fung and her colleagues found that increasing temperatures and drought frequencies lower plant uptake of CO₂ as plants breathe in less to conserve water. At some point, the rate of fossil fuel CO₂ emissions will outstrip the ability of the vegetation to keep up, leading to a rise in atmospheric CO₂, increased greenhouse temperatures and increased frequency of droughts. An amplifying loop leads to ever higher temperatures, more droughts, and higher CO₂ levels.

The oceans exhibit a similar trend, Fung said, though less pronounced. There, mixing by turbulence in the ocean is essential for moving CO₂ down into the deep ocean, away from the top 100 meters of the ocean, where carbon absorption from the atmosphere takes place. With increased temperatures, the ocean stratifies more, mixing becomes harder, and CO₂ accumulates in the surface ocean instead of in the deep ocean. This accumulation creates a back pressure, lowering CO₂ absorption.

In all, business as usual would lead to a 1.4° C (2.5° F) rise in global temperatures by the year 2050. This estimate is at the low range of projected increases for the 21st century, Fung said, though overall, the model is in line with others predicting large ecosystem changes, especially in the tropics.

With voluntary controls that flatten fossil fuel CO₂ emission rates by the end of the century, the land and oceans could keep up with CO₂ levels and continue to absorb at their current rate, the model indicates.

“This is not a prediction, but a guideline or indication of what could happen,” Fung said. “Climate prediction is a work in progress, but this model tells us that, given the increases in greenhouse gases, the Earth will warm up; and given warming, hot places are likely to be drier, and the land and oceans are going to take in carbon at a slower rate; and therefore, we will see an amplification or acceleration of global warming.”

“The Earth is entering a climate space we’ve never seen before, so we can’t predict exactly what will happen,” she added. “We don’t know where the threshold is. A two-degree increase in global temperatures may not sound like much, but if we’re on the threshold, it could make a big difference.”

Fung and colleagues have worked for several decades to produce a model of the Earth’s carbon cycle that includes not only details of how vegetation takes up and releases carbon, but also details of decomposition by microbes in the soil, the carbon chemistry of oceans and lakes, the influence of rain and clouds, and many other sources and sinks for carbon. The model takes into account thousands of details, ranging from carbon uptake by leaves, stems, and roots to the different ways that forest litter decomposes, day-night shifts in plant respiration, the salinity of oceans and seas, and effects of temperature, rainfall, cloud cover and wind speed on all these interactions.

All of today’s climate models are able to incorporate the climate effects of carbon dioxide in the atmosphere, but only with concentrations of CO₂ specified by the modelers. Fung’s model does not specify atmospheric CO₂ levels, but rather predicts the levels, given fossil fuel emissions. The researchers used observations of the past two centuries to make sure that their model is reasonable, and then used the model to project what will happen in the next 100 years.

The climate model coupled with the carbon cycle has been her goal for decades, as she tried to convince climate modelers that “whether plants are happy or not happy has an influence on climate projections. To include interactive biogeochemistry in climate models, which up to now embrace primarily physics and dynamics, is new.”

She admits, however, that much work remains to be done to improve modeling. Methane and sulfate cycles must be included, plus effects like changes in plant distribution with rising temperatures, the possible increase in fires, disease, or insect pests, and even the effects of dust in the oceans.

“We have created a blueprint, in terms of a climate modeling framework, that will allow us to go beyond the physical climate models to more sophisticated models,” she said. “Then, hopefully, we can understand what is going on now and what could happen. This understanding could guide our choices for the future.”

 

This article written by: NCAR Media Relations Office, Robert Sanders (UC Berkeley), Mario Aguilera and Cindy Clark (Scripps Institution of Oceanography), John Hules (Berkeley Lab).