Climate Variability
and Change
USGCRP Fiscal Year 2003 Accomplishments

Observed Arctic warming trend over the last 20 years:

Observations of Arctic-wide surface temperatures using satellite data have shown that over the period 1981-2001, the Arctic region warmed at an annual average rate of 0.3°C per decade over sea ice (considering those portions of the Arctic Ocean where 80% of ocean surface is covered by ice), 0.5°C per decade over the high latitude (poleward of 60 degrees North) region of Eurasia, and 1.0°C over the high latitude region of North America. Temperature trends derived from surface data are similar over much of the Arctic, but differ in some sub-regions. In comparison, during the last 20 years the global annual average surface temperature has increased by about 0.2°C per decade. At the high northern latitudes, the warming trends are more pronounced in the spring and are also evident in summer and fall, resulting in a longer melt season for snow and ice on land and for annual sea ice.

Satellite data also show that the portion of the Arctic Ocean covered by perennial sea ice has declined by about 9% per decade since 1978. The longer melt season and loss of perennial sea ice cover can have large-scale climate consequences. They permit an increase in the amount of energy absorbed in the previously ice- or snow-covered areas and, on land, permit increased growth of vegetation that also has a lower a (lower light reflectivity) than snow covered areas.

Climate models project that the high latitude regions are particularly sensitive to climate change because of the positive albedo feedback effects associated with reduction of ice and snow cover, and the reduction of thermal insulation of the ocean that sea ice cover provides, allowing increased heat transfer from the ocean to the atmosphere. However, there is as yet no direct evidence that greenhouse gas forcing, which drives the climate models, is responsible for the melting of sea ice and snow cover in the Arctic region.

The data also show regional differences that suggest there are other influences in addition to global-scale climate warming. A natural weather pattern called the North Atlantic Oscillation/Northern Annular Mode (NAO/NAM) may have contributed to regional variations as well as the overall decrease in Arctic sea ice cover over the last 20 years. Whether the ice cover as a whole will continue to exhibit the decreases that it experienced over the 1979 to 2003 period might depend on the strength and phase of the NAO/NAM, as well as on long-term trends in the climate system. (See Figures 8a and 8b)

Increasing ocean heat storage:

Simulations with an improved version of the NASA/GISS Global Climate Model indicate that the rate of heat storage by the world’s oceans has increased from about 0.2 watts/m² in 1951 to a present value of about 0.7 watts/m² net downward flux (convergence) of heat into the ocean surface. This is the third independent climate model to produce such an increase, and compares well with observational analyses of changes in ocean heat storage. Since the ocean stores a large portion of the excess heat due to the imbalance of the radiation budget of the Earth’s climate system, this work indicates that careful monitoring of the global distribution of ocean heat storage will be a key indicator for identifying changes in the climate system.

Change in the freshwater balance of the Atlantic Ocean:

The distribution of salinity in the Atlantic Ocean has been sampled over a broad area during the last half-century. These historical data can be used to diagnose rates of surface freshwater fluxes, freshwater transport, and local ocean mixing — important components of climate. Recent research comparing observed salinities on a long transect through the western basins of the Atlantic Ocean between the 1950s and the 1990s found systematic increases in freshwater at high latitudes (at both poleward ends), contrasted with large increases of salinity at low latitudes. Although the observational record is insufficient to quantify a number of factors that may have contributed to these long-term trends, a growing body of evidence suggests that shifts in the oceanic distribution of fresh and saline waters are occurring in ways that may be linked to global warming and possible change in the global water cycle. Parallel changes in ocean salinity and temperature are occurring in other oceans as well.

20th Century global sea-level rise:

The rate and causes of 20th century global sea-level rise are subjects of intense debate. Direct observations, based on tide gauge records suggest that the rate of sea-level rise is between 1.5 and 2.0 mm/year (0.6 to 0.8 inches/decade). The two largest contributors to sea-level rise are thought to be volume changes due to ocean warming (thermal expansion) and the addition of mass due to the melting of polar ice sheets, although the magnitudes of these contributions are not well known. Scientists at NOAA’s Laboratory for Satellite Altimetry analyzed tide gauge records, which reflect both volume and mass changes, and ocean temperature and salinity data, which reflect only volume changes, in the North Pacific and North Atlantic Oceans. They found that measurements of sea-level rise from tide gauges are 2-to-3 times higher than those from temperature and salinity measured regionally and near gauge sites. The data support earlier estimates of the 20th century rate of sea-level rise and, more importantly, also provide the first evidence suggesting that addition of mass due to the melting of polar ice sheets can play an important, perhaps dominant, role in sea-level rise.

Simulating 20th century climate:

Multiple ensemble simulations of the 20th century climate have been conducted using climate models that include new and improved estimates of natural and anthropogenic forcing. The simulations show that observed globally averaged surface air temperatures can be replicated only when both anthropogenic forcings, e.g., greenhouse gases, as well as natural forcings such as solar variability and volcanic eruptions are included in the model. These simulations improve on the robustness of earlier work. Comparisons of the model results with observations indicate that regionally concentrated increases in precipitation can occur as a function of variability in solar forcing. (See Figure 9)

Detecting a human influence on North American climate:

A recent study shows that the average global results reported above also pertain over the North American region. Several indices of large-scale patterns of surface temperature variation were used to investigate climate change in North America over the 20th century. The observed variability of these indices was simulated well by several climate models. Comparison of index trends in observations and model simulations shows that North American temperature changes from 1950 to 1999 were unlikely to be due only to natural climate variations. Observed trends over this period are consistent with simulations that include anthropogenic forcing from increasing atmospheric greenhouse gases and sulfate aerosols. However, most of the observed warming from 1900 to 1949 was likely due to natural climate variation.

Long-term drought reconstructions for North America:

Tree-ring paleo-proxy records have been used to develop an animated atlas of North American drought for the last ~1,000 years. The data show annual (and even within-year) resolution of drought/wetness conditions across the United States and parts of Mexico and Canada. This synthesis provides a dramatic visual representation of changing climatic and environmental conditions over the region, including an indication that significantly more arid conditions existed in parts of the western United States prior to AD 1500. Such paleoclimate data help aid the understanding of climate mechanisms and impacts.

Origins of recent severe droughts in the Northern Hemisphere:

Recent work provides compelling evidence that severe droughts that affected the United States, the Mediterranean region, and Southwest Asia simultaneously during 1998-2002 were part of a persistent climate state that was strongly influenced by the tropical oceans. The oceanic conditions of importance were unusually cold sea surface temperatures (SSTs) in the eastern tropical Pacific, i.e., persistent La Niña conditions, that occurred together with sustained above normal SSTs in the western tropical Pacific and Indian Oceans. The persistence of this abnormal tropical SST pattern was unprecedented in the instrumental record. A large suite of model simulations showed that this SST pattern was ideally suited to force atmospheric circulation anomalies that were conducive to producing abnormally dry conditions in those regions where severe and sustained drought was observed.

Causes of the 1930s Dustbowl:

A NASA atmospheric general circulation model was used to investigate the North American dustbowl drought during the 1930s. Ensemble simulations using observed sea-surface temperatures (SSTs) show that principal causes of the Great Plains drought were the anomalous tropical SSTs during the 1930s in the Pacific and, to a lesser extent, the Indian and Atlantic Oceans. Land-surface feedbacks were also essential to the development and maintenance of the severe drought conditions.

Role of stratosphere:

Recent observational analyses suggest that, together with the tropical oceans, the stratosphere increases the ‘memory’ of the climate system, and also may influence long-term variations in the polar ice pack, sea surface temperatures, and the deep ocean circulation. This stratosphere-troposphere connection has important implications for the prediction of the response of tropospheric climate under increasing concentrations of greenhouse gases. Currently, sophisticated climate models differ as to whether the stratospheric polar vortex, a key part of the connection, will strengthen or weaken with increasing concentrations of greenhouse gases.

Role of aerosol infrared forcing:

A crucial factor limiting the predictability of global climate is the large uncertainty about the precise effects of aerosols on Earth's radiation balance. Most large-scale global climate models include the direct radiative effects of aerosols at higher wavelengths, but few consider aerosol radiative properties in the infrared (IR) region. Measurements of clear-sky IR spectra, performed during a cruise across the western Pacific Ocean, revealed aerosol forcings of up to 10 W/m². These values are quite large compared to the 1-2 W/m² forcing estimated for greenhouse gas accumulations since the beginning of the industrial revolution. Based on these measurements and analyses, aerosol IR effects will be included in the next version of the National Center for Atmospheric Research (NCAR) Community Climate System Model.

Effects of Indo-Pacific ocean mechanisms:

A new multi-year assimilation of in-situ and satellite data into an ocean model highlighted the importance of the interior ocean mechanisms (as compared to boundary currents such as the Gulf Stream) on timescales of weeks to months. Investigators found these mechanisms in the interior ocean play a critical role in altering the water mass exchanges between the midlatitude eastern Pacific Ocean and the the tropical Pacific where El Niño develops, suggesting that remote effects on El Niño should be more carefully considered by prediction models. Further, these relatively fast mechanisms were found to govern more generally the transports and exchanges between the tropical and midlatitude ocean and thus could be an important factors for observing and modeling the longer-term changes (e.g., interannual to decadal variability) of the Pacific Ocean.

Diagnostic for Evaluating Climate Model Performance :

Scientists developed the Broadband Heating Rate Profile (BBHRP), a new model diagnostic that will help reduce a significant obstacle to improving the predictive accuracy of climate models—the ability to accurately quantify the interaction of the clouds, aerosols, and gases in the atmosphere with radiation. Because direct observation of these interactions is extremely difficult, there has been no observation standard with which to compare and judge the accuracy of climate model simulations. The BBHRP, which is based on an assimilation of detailed field measurements from the Atmospheric Radiation Measurement program, provides a realistic estimate of radiative heating or cooling impact of clouds, aerosols, and gases. This diagnostic can be directly compared to the model-predicted impacts, thus enabling model uncertainties to be evaluated.