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Climate Variability and Change
The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2009 edition of the annual report, Our Changing Planet).
This year’s highlights of climate variability and change research emphasize advances in several areas of high societal interest. These include improving understanding of tropical cyclone-climate links; droughts; the predictability of the El Niño-Southern Oscillation; changes in the North Atlantic Ocean circulation; ice processes and implications for sealevel changes; the causes of sea ice declines; and detection and attribution of the causes for regional climate changes, including effects of air pollution on Asian climate.
Assessing Tropical Cyclone-Climate Links from Paleoclimate Data.1,2 A new line of research is applying paleoclimate data to study tropical cyclone behavior before the period of modern observational records, a field called “paleotempestology.” This research provides the potential to study climate-tropical cyclone relationships through a wider range of climates than can be obtained from the modern observational period. By greatly increasing the time interval under consideration, this approach also enables the possibility of detecting rare, catastrophic events that have very long recurrence intervals (hundreds of years or longer). Using marine sediment cores and corals, two recent studies have quantified the frequency of major hurricanes in the North Atlantic over the past 5,000 years. They find that the frequency of major hurricanes has varied on centennial to millennial time scales; however, a simple relationship to sea surface temperature variations seems to be lacking. These studies suggest the potential importance of other factors, such as variations in the El Niño-Southern Oscillation, tropical monsoons, and trade wind strength in determining variations in the strength of hurricanes.
Climate-Hurricane Links Over the North Atlantic.3,4,5,6 Vigorous scientific debate continues about the detection of trends in Atlantic hurricane activity, with several studies published within the past year giving conflicting findings. One recent study showed a strong positive correlation between multi-decadal sea surface temperature variations and tropical cyclone counts and concluded that a trend toward increasing numbers of tropical cyclones in the Atlantic basin has been driven predominantly by anthropogenically forced increases in Atlantic sea surface temperatures. In contrast, another study suggested that the estimated increase in the number of Atlantic hurricanes over the 20th century is mostly, if not entirely, due to observational uncertainties that lead to substantial storm undercounts early in the period. Yet another study argues that a trend toward increasing Atlantic hurricane activity is insensitive to a range of estimates of storm undercounts. Efforts continue to reanalyze the tropical cyclone record, as well as the climate of the early part of the 20th century. This work should improve ability to describe and understand the causes of climate-hurricane relationships.
Modeling Studies of Climate Factors Influencing Hurricanes.7,8 Recent research on Atlantic hurricanes and climate change has focused on whether the increase in hurricane activity in the basin since the 1970s portends future large increases in a warming climate. In an analysis of projected climate changes over the tropical Atlantic region during the 21st century derived from 18 different climate models developed for the IPCC Fourth Assessment Report, a notable finding was that the vertical wind shear (the difference in wind direction and speed between the lower and upper atmosphere) is projected to increase across much of the Caribbean in the warmer climate, a factor that tends to suppress tropical storm and hurricane development and intensification. In other basins, shear was reduced, which would tend to favor enhanced tropical storm activity. This study emphasizes the importance of incorporating changes in vertical wind shear as well as sea surface temperatures in projections of future hurricane activity. A second study introduced a new regional modeling approach for understanding mechanisms that produce changes in Atlantic hurricane activity. The study uses a high-resolution model (18-km, or about 11-mile grid spacing) in which the large-scale atmospheric conditions are provided from historical analyses (“reanalyses”) and ocean conditions are derived from observed sea surface temperatures. Within the evolving large-scale state, the model generates hurricane activity. This method successfully models the observed increase in Atlantic hurricane activity over the period 1980 to 2006, as well as other features like diminished hurricane activity during El Niño years. This new approach should provide an important means for addressing key questions regarding possible responses of Atlantic hurricane activity to future climate warming.
Climate Model Projections of Increasing Aridity in Southwestern North America.9 A recent study examines how the hydroclimate over southwestern North America may change over the remainder of this century. The study evaluates climate change projections out to 2100 derived from 19 climate models used in the IPCC Fourth Assessment Report. These models were forced with the A1B emissions scenario, in which carbon dioxide (CO2) emissions increase until about 2050 and then decrease modestly. The study shows that 18 of the 19 models predict a drier climate for the southwest United States. The projected trend in “precipitation minus evaporation,” an indicator of drought potential (see Figure 6), implies that droughts as severe as the 1930s Dust Bowl drought may become common occurrences in this region within the next few decades. The projections suggest that rain-producing storms that normally affect the southwest United States will shift northward, making the region drier. Further work will be required to understand the reason for the shifts in the storm patterns.
Seasonal-to-Interannual Predictability of the Tropical Pacific and Atlantic.10,11,12 A debate currently exists about the extent to which the predictability of the El Niño-Southern Oscillation (ENSO) is limited by uncertainties in specifying initial conditions or by errors associated with “weather noise” and other unresolved physical processes. Understanding what limits climate predictability is critical to quantifying confidence in ENSO predictions and related impacts over North America. Recent research, using a new interactive ensemble coupling strategy, suggests the skill of current operational ENSO forecasts is limited mainly by initial condition errors. Other sources of error, however, do have a significant impact on the predictability of La Niña events and large El Niño events. For the tropical Atlantic sector, research is exploring the impact of biases in model simulations on their ability to reproduce and predict the leading patterns of climate variability in this region, with the goal of improving operational seasonal forecasts.
New Observations of the North Atlantic Ocean Circulation.13,14 An issue of major concern is the potential for rapid and dramatic collapse of the northward flow of warm water in the North Atlantic, which may have serious implications for the climate of surrounding regions, including North America and Europe. A new international observing program, partly supported by U.S. agencies, is examining whether and how this flow is changing. The northward flow of relatively warm, near-surface water is closely linked to a southward flow of cold, deep water, which forms an oceanic conveyer belt called the Atlantic Meridional Overturning Circulation (AMOC). Two recent studies report on observations of the variability in the AMOC, which is a fundamental prerequisite to understanding trends in this circulation, as well as its potential for rapid collapse. The studies indicate surprisingly large variability over the course of the 1-year period for this new program, suggesting that trends detected in previous studies from “snapshot” observations may be strongly influenced by the sampling of this large intra-annual variability. Observations from this program appear to be of sufficient accuracy to detect any large abrupt transition of the AMOC. The short period of record precludes a description of how the AMOC varies interannually. Understanding this variability will be critical for improving climate models used in predictions and projections of climate variability and change.
Ice Processes and Sea Level Change.15,16,17 Future sea-level rise will depend on how much the ocean expands from future warming, as well as ice loss to the sea from glaciers, ice caps, the larger Greenland and Antarctic Ice Sheets, and other factors. A recent study found that about 60% of the ice loss in recent years has been from glaciers and ice caps (features whose perennial snow and ice coverage is less than 50,000 km2) rather than the larger ice sheets. Acceleration of glacier melt, in part due to processes not yet considered in models, may cause an additional 0.1 to 0.25 m of sea-level rise by 2100. An analysis of Special Sensor Microwave/Imager (SSM/I) radiometer measurements over the Greenland Ice Sheet back to 1988 shows that there was an extended period of melting snow in 2006, with snowmelt occurring more than 10 days longer than the 1988-2005 average over certain areas. Yet another separate analysis using satellitederived surface elevation data found major variations in recent ice discharge and mass loss at two of Greenland’s largest outlet glaciers. These studies point to the need to better constrain observations and advance understanding of ice outlet dynamics in order to improve projections of future sea-level rise. As noted in the IPCC Fourth Assessment Report, there are presently large uncertainties about the processes controlling ice flows, and advances in this area will be critical to improving models required for estimating future sea-level rise.
Declines in Arctic Sea Ice.18,19 There have been dramatic declines in Arctic sea ice in recent decades that are particularly strong in summer and early fall (Figure 7), but are evident throughout the year. A study compared observations against sea ice simulations from more than a dozen models that were used in the IPCC Fourth Assessment Report. The results indicated that nearly all the models failed to produce as much of a decrease in minimum Arctic sea ice coverage as was observed, often substantially underestimating the decreases. The authors suggest that the Arctic could be seasonally free of sea ice earlier than IPCC projections. Another study considered factors that may contribute to declines in sea ice in the Arctic winter. The results show important regional differences in the processes leading to the changes, although the hemispheric-mean decline in winter ice extent appears primarily due to increasing sea surface temperatures in the Barents Sea and adjoining waters. This result is in contrast to earlier work showing that downwelling longwave radiation is the leading factor contributing to changes in Arctic minimum sea ice extent in late summer and early fall, although relative contributions from different factors vary by region and year. The results emphasize the important roles of both atmospheric and oceanic processes in understanding and modeling the causes of rapid sea ice declines.
Detection of Human Influence on 20th Century Precipitation Trends.20,21,22 Human influence on climate has been detected in numerous quantities, but changes in precipitation have been more difficult to attribute to human causes, partly due to sampling errors associated with inferring global precipitation trends from the archive of mostly landbased measurements. Two recent studies shed new light on this detection problem. One study makes use of monthly precipitation observations over global land areas for 50 to 75 years prior to 2000 and demonstrates that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands. In particular, anthropogenic forcing has contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. These opposing zonal trends help explain why detecting and explaining a net global trend has been so challenging. Satellite observations of precipitation reduce sampling errors but are available for a more limited period of time. In a second study, satellite observations are used to show a globally averaged increase in evaporation, precipitation, and total water of about 1.3% per decade for the period July 1987 to August 2006. During that time period, satellite measurements showed a positive trend in the globally averaged surface and lower atmosphere temperatures of about 0.2°C per decade, implying an increase in evaporation, precipitation, and total water of about 6.5% per °C. These increases in evaporation and total water agrees with climate models, but the models predict that global precipitation will increase at a much slower rate of 1-3% per °C. The reason for the discrepancy between satellite observations of precipitation and models is not clear.
Identifying a Human “Fingerprint” Pattern in Atmospheric Water Vapor Changes.23 Water vapor feedbacks are likely to play a key role in determining the magnitude of climate changes over the next century. Data from the satellite-based SSM/I show that the total atmospheric moisture content over oceans has increased by 0.41 kg m-2 per decade since 1988. Current climate models indicate that water vapor increases of this magnitude cannot be explained by natural variability alone. In a formal detection and attribution analysis using the pooled results from 22 different climate models, the simulated “fingerprint” pattern of anthropogenic changes in water vapor is identifiable with high statistical confidence in the SSM/I data. Experiments have been conducted in which forcing factors related to greenhouse gases, solar radiation, and volcanic eruptions are varied individually. The study found a decrease in water vapor levels due to cooling induced by massive volcanic eruptions (e.g., Pinatubo in 1991). The “fingerprint match” of the observed levels of water vapor indicates that the increases are primarily due to human-caused increases in greenhouse gases and not to solar or volcanic forcing. The findings provide evidence of an emerging anthropogenic signal in the moisture content of Earth’s atmosphere.
Assessing the Causes of Global Stratospheric Temperature Changes.24 The substantial cooling of the global lower stratosphere between 1979 and 2003 occurred in two pronounced step-like transitions. These arose in the aftermath of two major volcanic eruptions, with each cooling transition followed by a period of relatively steady temperatures. Climate model simulations demonstrate that the space-time structure of the observed cooling is largely attributable to the combined effect of changes in both anthropogenic factors (ozone depletion and increases in well-mixed greenhouse gases) and natural factors (solar irradiance variation and volcanic aerosols). The anthropogenic factors drove the overall cooling during the period, while the natural ones modulated the evolution of the cooling. The quantitative understanding of the global-mean lower stratospheric temperature evolution, including the distinct roles of the different forcing agents, is now at a high confidence level.
Black Carbon Impacts on the Asian Monsoon.25 There is a growing awareness that black carbon aerosols, with their properties of both absorbing and reflecting solar radiation, may be contributing to significant climate change in the Indian monsoon region of south Asia. To address this problem, a six-member ensemble of 20th-century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model was analyzed to study the effects of black carbon aerosols on the Indian monsoon. The black carbon aerosols increase lower tropospheric heating over south Asia and reduce the amount of solar radiation reaching the surface during the dry season. The increased north-south tropospheric temperature gradient in the pre-monsoon months of March, April, and May, particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months. With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau. During the summer monsoon season, the model experiments show that black carbon aerosols have likely contributed to observed decreasing precipitation trends over parts of India, Bangladesh, Burma, and Thailand.
Potential Impacts of Future Temperature Changes on Tropical Glaciers.26,27,28 Recent studies have considered the potential impact of future climate warming on tropical glaciers in the Americas, which provide a vital source of fresh water (as well as hydropower) for local communities. One study examines projected changes in mean annual free-air temperatures between 1990 to 1999 and 2090 to 2099 along a section of the American Cordillera from Alaska to Chile. The results show a general tendency toward increased warming at higher altitudes, with maximum temperature increases projected to occur in the high mountains of Peru, Bolivia, and northeast Chile. Such changes would have major consequences for mountain glaciers and for communities that depend on glacier meltwater for their water supplies. Because the Andean region has become highly dependent on hydropower (comprising over 50% of total energy supply in Ecuador and about 80% in Peru), rapid glacier retreat in the Andes also has substantial economic implications for the region.
Additional Past Accomplishments:
CLIMATE VARIABILITY AND CHANGE CHAPTER REFERENCES
8) Knutson, T.R., J.J. Sirutis, S.T. Garner, I.M. Held, and R.E. Tuleya, 2007: Simulation of the recent multi-decadal increase of Atlantic hurricane activity using an 18-km grid regional model. Bulletin of the American Meteorological Society, 88, 1549-1565.
9) Seager, R., M. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H.-P. Huang, N. Harnik, A. Leetmaa, N.-C. Lau, C. Li, J. Velez, and N. Naik, 2007: Model projections of an imminent transition to a more arid climate in southwestern North America. Science, 316, 1181-1184.
10) Stan, C. and B.P. Kirtman, 2008: The influence of atmospheric noise and uncertainty in ocean initial conditions on the limit of predictability in a coupled GCM. Journal of Climate, 21, doi:10.1175/2007JCLI2071.1.
13) Cunningham, S.A., T. Kanzow, D. Rayner, M.O. Baringer, W.E. Johns, J.Marotzke, H.R. Longworth, E.M. Grant, J.J.-M. Hirschi, L.M. Beal, C.S. Meinen, and H.L. Bryden, 2007: Temporal variability of the Atlantic meridional overturning circulation at 26.5° N. Science, 317, 935-938.
14) Kanzow, T., S. Cunningham, D. Rayner, J.J.-M. Hirschi, W.E. Johns, M.O. Baringer, H.L. Bryden, L.M. Beal, C.S. Meinen, and J.Marotzke, 2007: Observed flow compensation associated with the MOC at 26.5°N in the Atlantic. Science, 317, 938-941.
15) Meier, M.F., M.B. Dyurgerov, U.K. Rick, S. O’Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson, and A.F. Glazovsky, 2007: Glaciers dominate eustatic sea-level rise in the 21st century. Science, 317, 1064-1066.
20) Zhang, X., F.W. Zwiers, G.C. Hegerl, H.F. Lambert, N.P. Gillett, S. Solomon, P.A. Stott, and T. Nozawa, 2007: Detection of human influence on twentieth-century precipitation trends. Nature, 448, 461-466.
23) Santer, B.D., C. Mears, F.J. Wentz, K.E. Taylor, P.J. Gleckler, T.M.L. Wigley, T.P. Barnett, J.S. Boyle, W. Brüggemann, N.P. Gillett, S.A. Klein, G.A. Meehl, T. Nozawa, D.W. Pierce, P.A. Stott, W.M. Washington, and M.F. Wehner, 2007: Identification of human-induced changes in atmospheric moisture content. Proceedings of the National Academy of Sciences, 104(39), 15248-15253.
24) Ramaswamy, V., M.D. Schwarzkopf, W.J. Randel, B.D. Santer, B.J. Soden, and G.L. Stenchikov, 2006: Anthropogenic and natural influences in the evolution of lower stratospheric cooling. Science, 311, 1138-1141.
27) Thompson, L.G., E. Mosley-Thompson, H. Brecher, M. Davis, B. León, D. Les, P-Nan Lin, T. Mashiotta, and K. Mountain, 2006: Abrupt tropical climate change: past and present. Proceedings of the National Academy of Sciences, 103, 10536-10543.
28) Vergara, W., A. Deeb, A. Valencia, R.S. Bradley, B. Francou, A. Zarzar, A. Grunwaldt, and S. Haeussling, 2007: The economic impacts of rapid glacier retreat in the Andes. Eos,Transactions of the American Geophysical Union, 88, 261, 264.