Climate Variability
and Change
USGCRP Fiscal Year 2006 Accomplishments

The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2007 edition of the annual report, Our Changing Planet).

The highlights of recent research on climate variability and change presented here are generally arranged by time scale, extending from paleoclimate through more recent climate characterizations. Additional subsections illustrate important progress in understanding fundamental climate processes and in identifying projected changes. The topics “Climate Model Evaluation, Diagnosis, and Improvement” and “Detection and Attribution of Climate Change” are given special attention in this section.

Paleoclimate

This section begins with an example of the use of paleoclimatic (i.e., prehistoric climate) data to improve understanding of natural climate variability.

High-Resolution Records of Prehistoric Climate [4, 10, 30, 37, 42, 44, 56]

Researchers have developed new high-resolution data records detailing paleoclimatic variations in ancient terrestrial and marine environments. Many of the records span the last ten to twenty thousand years, and some extend back several million years. These records, which are based on a variety of climate proxies (e.g., tree rings, pollen, fossil shells, foraminifera, diatoms), are being used to document multi-decadal and longer term trends in temperature and precipitation. This information is also being used to evaluate climate system variability, such as the influences of solar variability, changes in the position of the Intertropical Convergence Zone (ITCZ), and effects of large-scale climate patterns and processes (e.g., the North Atlantic Oscillation and El Niño) on regional climate. New data from rapidly accumulating sediments have also provided evidence of abrupt climate changes in the past related to changes in deep ocean circulation and other processes. Many of the records are the results of published scientific research contributed to the World Data Center for Paleoclimatology, operated by the National Climatic Data Center. For proxy information relevant to surface temperature reconstructions over the last 1,200 years or so, considerable effort has gone into testing the sensitivity of the reconstructions to the methods used. Recent results are in general agreement with previously published hemispheric-scale reconstructions and suggest that further improvements will depend more on the quality, rather than the quantity, of available proxy data. This conclusion and, more generally, the state of the science in reconstructing surface temperature records over the past one to two millennia, was the topic of a recent report by a panel of the National Research Council (NRC).

Recent Observed Changes

The following research highlights illustrate some recently observed climate system changes.

CCSP Synthesis and Assessment Report on Atmospheric Temperature Trends [6, 31, 45, 48]

Taking advantage of new surface, satellite, and radiosonde data and new model simulations of 20th century climate, CCSP Synthesis and Assessment Product 1.1 addresses temperature changes from the surface through the lower stratosphere, differences in these changes at various levels, and our understanding of the causes of these changes and differences. It assesses progress made since production of reports by the NRC and the Intergovernmental Panel on Climate Change (IPCC) in 2000 and 2001, and highlights several fundamental uncertainties and differences between and within the individual components of the existing observational and modeling systems. It is particularly relevant to the CCSP Goal 1 focus on increasing confidence in the understanding of how and why climate has changed. The study focuses on the period since 1958, when upper-air soundings by balloon-borne instruments started to become widespread, and on the period since 1979, when the satellite era of atmospheric temperature sounding began. Conclusions include the following:

• Surface Temperatures – For global-mean changes, as well as in the tropics (20°S to 20°N), all data sets show warming at the surface since 1958, with a greater rate of increase since 1979. The global average surface warming from 1958 to the present was 0.12°C per decade and the warming from 1979 to the present was 0.16°C per decade. Differences between various surface data sets are small.
• Tropospheric Temperatures – Global-mean temperature in the troposphere (the lower 10 km of the atmosphere) increased at about 0.14°C per decade since 1958, and between 0.10°C and 0.20°C per decade since 1979. In the tropics, temperature increased at about 0.13°C per decade since 1958, and between 0.02°C and 0.19°C per decade since 1979. All data sets show that the global-mean and tropical troposphere has warmed from 1958 to the present, with the warming in the troposphere being slightly more than at the surface. However, due to considerable disagreements between tropospheric data sets since 1979, it is not clear whether the troposphere has warmed more or less than the surface in the past two and half decades. Evidence suggests that non-climatic influences remaining in some or all of the observed tropical tropospheric data sets lead to biased long-term trends and are responsible for the large range in the observed values.
• Lower Stratospheric Temperatures: All data sets show that the stratosphere has cooled considerably from both 1958 and 1979 to the present, although there are large differences in the linear trend values from different data sets. Stratospheric cooling is expected from an increase in greenhouse gases. The largest differences between data sets are in the stratosphere, particularly between the radiosonde and satellite-based data sets. It is very likely that the satellite-radiosonde discrepancy arises primarily from uncorrected errors in the radiosonde data.

When state-of-the-art climate models are run with natural and human-induced forcings, simulated global-mean temperature trends for individual atmospheric layers are consistent with observations. Comparing trend differences between the surface and the troposphere exposes discrepancies between model simulations and observed data in the tropics. In the tropics, the majority of observational data sets show more warming at the surface than in the troposphere, while almost all model simulations have larger warming aloft than at the surface.

Several research efforts were catalyzed in part by the production of this synthesis and assessment product during the research community’s input to its early stages. One of these studies concluded that an error in the satellite data associated with the day-night temperature cycle reduces the observed warming aloft. Another study concluded that changes in instrumentation of balloon-borne sensors have introduced a spurious cooling anomaly in upper air temperatures measured by those sensors. When these observational factors are accounted for, the observations and models come into closer agreement.

Melting Sea Ice [9,38,53,59]

On September 21, 2005, NASA and the National Snow and Ice Data Center observed the lowest extent of Arctic sea ice (5.3 million km2) in the satellite record, which extends back to 1978 (see Figure 5). This brings the estimated decline in perennial Arctic sea ice to 9.8% per decade over the satellite record. The period 2002-2005 showed ice extents that were approximately 20% below the 1978-2000 average—a reduction in area of 1.3 million km2 (or twice the size of Texas). The persistence of near-record low sea-ice extent raises concern that Arctic sea ice may be in a continual, long-term decline. Models driven by projected increases in greenhouse gases project a decrease in summer sea ice of more than 50% over the 21st century, although such projections should be tempered by the recognition that simulations of present-day sea ice generally differ from observed seasonal and geographical distributions. According to paleoclimatic records, there is no evidence of an ice-free summer Arctic during the last 800 millennia.

Declining North Atlantic Salinity [11]

An analysis of oceanographic salinity data indicates that large amounts of freshwater have been added to the northern North Atlantic Ocean since the mid-1960s. This work quantified for the first time the volume and time evolution of this freshwater intrusion as well as its extension into the Nordic Seas and subpolar basins. The pattern of freshwater accumulation observed in the Nordic Seas suggests it would take nearly a century to reach freshening thresholds that could abruptly shut down the primary processes governing the meridional overturning (thermohaline) circulation in the North Atlantic. This circulation pattern, which consists of a northward surface component and a southward deep ocean component, is particularly important because it helps maintain relatively moderate temperatures in western Europe, among other things (see schematic diagram in Figure 6).

Fewer Days of Ice on Northern New England Rivers [20]

The total number of days of ice on northern New England rivers has declined significantly in recent decades, particularly in the spring. In a recent study, hydrologists examined data from streamflow gauging stations in Maine, New Hampshire, and Vermont that measure the height and flow of rivers. They examined the number of days each year of ice-affected flow—that is, days when there is enough ice in a river to affect the relation between the height and the flow of the river. They found that the number of ice-affected flow days decreased significantly during the 20th century for 12 of the 16 rivers studied. The total days of ice-affected flow decreased by an average of 20 from 1936 to 2000, with most of the decrease occurring since the 1960s. On average, ice-out dates were 11 days earlier in 2000 compared to 1936, again with most of the change occurring since the 1960s. The changes are consistent with warming temperatures in the late winter and spring in New England during the last 30 to 40 years. Another study of lakes in the region showed that ice-out dates were approximately 5 days earlier in 2000 compared to 1968 in northern and mountainous areas of Maine and New Hampshire and approximately 13 days earlier in more southerly areas of these states.

Hurricane Intensity Trends [13, 14, 23, 40, 41, 54]

A pair of studies suggests a global average increase in the overall intensity, but not frequency, of hurricanes over approximately the last three decades. One study found a trend toward increasing potential destructiveness of tropical storms, as measured by “total power dissipation” measured over the lifetime of the storms. Changes in potential destructiveness were highly correlated with long-term trends in tropical sea surface temperatures (SSTs) over this period. A second study found a large increase in the number and proportion of hurricanes reaching very intense status, as defined by Saffir-Simpson categories 4 and 5, with the greatest increases occurring in the North Pacific, Indian, and southwest Pacific Oceans. As in the first study, the intensity increases are correlated with long-term increases in tropical SSTs observed in the global oceans. Interestingly, the overall numbers of tropical cyclone days have decreased in all of the global ocean basins except the North Atlantic over the past few decades. While these studies are consistent, the results do not definitively link the trends in tropical cyclone intensity to global warming, and some experts caution that deficiencies in past observations of hurricanes preclude confident identification of trends over this period. Other work has emphasized the vital importance of changing demographics and land use in coastal regions in increasing societal vulnerability to these storms, independent of any changes in hurricane intensity or frequency.

Fundamental Understanding of Climate Processes

Much of the work within the CVC research element focuses on improving fundamental understanding of climate processes. The following research highlights are examples of this type of work.

Vegetation-Climate Feedback [28, 35]

While it is well known that climate is an important driver of vegetation growth, less is known about the potential influence of vegetation on climate. To date, two-way vegetation-climate interactions (i.e., feedbacks) have been mostly studied using computer models with minimal attempt to investigate observed feedbacks. Recent studies have made initial attempts to quantify the impacts of vegetation variability on climate using observed data. One of these studies assessed observed vegetation feedbacks on surface air temperature and precipitation across the United States using satellite-derived vegetation data and observed monthly climate data for 1982-2000. The results show that an increase in vegetation generally leads to a substantial increase in temperature across the northern states, particularly in the spring. The impact of vegetation on precipitation appears to be more complex and relatively weak. It appears that an increase in vegetation over the major croplands of the United States supports an increase in precipitation. Figure 7 illustrates these responses.

Understanding and Modeling Ocean Mixing Processes [7, 15, 25, 46]

Ocean mixing processes that are too small to be explicitly included in current climate models are an important area of research, since these processes largely determine the rate of heat uptake by the ocean. Two U.S. Climate Variability and Predictability (CLIVAR) Climate Process and Modeling Teams (CPTs), supported by NSF, NOAA, and NASA, have focused on improving the understanding and representation of ocean mixing processes in climate models. One CPT is working on how to include in climate models the ocean mixing that results when dense waters flow over steep ocean bottom features, mixing with overlying waters. One region where this occurs is in
the North Atlantic (see the research highlight above on “Declining North Atlantic Salinity”) where waters from the Arctic region sink and pass south into the Atlantic and help form the meridional overturning circulation. Comparisons of modeling approaches have been completed and evaluations of new treatments of these processes in the models are underway. The second ocean CPT is studying the transfer and mixing of ocean properties by small upper ocean features. These small eddies transport large amounts of heat in the Southern Ocean and in the Gulf Stream and Kuroshio Current regions and modulate the exchange of heat between the ocean and atmosphere in these regions. A recent study shows that “salt fingers,” or filaments of salty water that sink in the ocean, play a more important role in vertical mixing of ocean waters than previously thought.

Projected Changes

The following highlights illustrate recent studies that have projected potential changes in future climate conditions based on simulations by state-of-the-art computer models.

Rainfall Extremes [18,55]

A set of recent modeling studies using the archived results of the IPCC Fourth Assessment Report models indicates that what would be considered a rare weather event at present may become commonplace by the end of this century. Furthermore, these rare events may increase in severity far more than changes in annual average conditions. For example, one study indicates that rainfall on the wettest day of the year is likely to increase more than changes in the average annual rainfall under conditions of increased greenhouse gases. Changes in such rainfall extremes may have larger impacts on the environment and society than changes in average annual rainfall. The results also suggest that the signature of greenhouse gas increases may be identified more easily from changes in extreme rainfall than changes in average rainfall.

Estimating Future Changes in Permafrost [24]

Analysis of 21st-century climate change simulations with a state-of-the-art climate model (Community Climate System Model, CCSM3) has shown that areas of permafrost may greatly decrease over the course of the 21st century, with the area of decrease proportional to the amount of warming, which in turn depends in part on the emissions scenario used in the model. Such large changes in permafrost may provoke positive feedbacks such as activation of the soil carbon pool and a northward expansion of shrubs and forests.

Tropical Ocean Response to Global Warming [27, 52]

Many studies have suggested that tropical Pacific SSTs are most likely to respond to global warming with an El Niño-like pattern, characterized by stronger warming in the east than in the west. However, a new finding from the most recent IPCC model simulations as well as past climate records challenges this traditional view. A recent study suggests that the most robust fingerprint of tropical Pacific SSTs in response to global warming is the so-called Enhanced Equatorial Response, characterized by an enhanced equatorial warming relative to the subtropics (Figure 8a). In comparison, the east–west difference in equatorial SSTs shows little systematic long-term change (Figure 8b). This new finding calls for a rethinking of the mechanism for tropical ocean response to global warming.

Climate Model Studies with Static Greenhouse Gas Concentrations [34]

A greenhouse gas stabilization experiment performed with two global climate models (the Parallel Climate Model and the CCSM3) shows that even if concentrations of greenhouse gases could be stabilized at present-day values, the thermal inertia of the climate system would lead to further warming, and ongoing sea-level rise due to thermal expansion of seawater. These modeling results indicate that if greenhouse gas concentrations were stabilized at present levels, by 2100 sea level may rise about three times the amount that has already been observed (see Figure 9), with continued sea-level rise for a few subsequent centuries.

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