GC24A-01 INVITED
Towards attributing climate change to causes on regional scales
The IPCC 4rth Assessment report concluded that greenhouse gas forcing has likely contributed to observed changes of temperature averaged over individual continents except Antarctica. However, it also points at present limitations in our ability to go beyond attributing climate change at continental scales. This talks discusses the evidence for changes attributable to anthropogenic forcing at continental and regional scales that was available at AR4 and evidence that has become available since. Difficulties in understanding and attributing climate change at smaller scales are discussed. These include the increasing importance of small-scale forcings such as land use change and aerosol forcing for regional trends, and the influence of modes of climate variability on regional scales. These difficulties get exacerbated when moving from temperature changes to other, possibly more important variables, such as changes in rainfall. Some examples for regional changes are discussed.
GC24A-02 INVITED
Attribution of the Regional Patterns of North American Climate Trends
North American trends in surface temperature and precipitation during 1951-2006 exhibit large spatial and seasonal variations. We seek to explain these by synthesizing new information based on existing model simulations of climate and its forcing, and based on modern reanalyses that describe past and current conditions within the free atmosphere. The presentation focuses on current capabilities to explain the spatial variations and seasonal differences in North American climate trends. It will address whether various heterogeneities in space and time can be accounted for by the climate system's sensitivity to time evolving anthropogenic forcing, and examines the influences of non-anthropogenic processes. New findings are presented that indicate anthropogenic forcing alone was unlikely the cause for key regional and seasonal patterns of change, including the absence of summertime warming over the Great Plains of the United States, and the absence of warming during both winter and summer over the southern United States. Key regional features are instead attributed to trends in the principal patterns of atmospheric flow that affect North American climate. It is demonstrated that observed variations in global sea surface temperatures have significantly influenced these patterns of atmospheric flow.
GC24A-03 INVITED
Human Induced Changes in the Hydrological Cycle of the Western United States
INVITED Numerous studies have documented marked change in components of the hydrological cycle of the western United States over the last 50 or so years. The snow pack has decreased and been observed to melt earlier in the calendar year. Main spring river runoff has also been coming earlier in the year over the entire region, while air temperatures, especially in Spring, have increased. Previous studies have speculated that such changes are due to global warming. However, strong interannual variability and marked decadal fluctuations in weather patterns greatly affect the region, and previous studies have not performed a formal detection and attribution analysis of these changes, nor attempted to quantify the comparative effect of natural and anthropogenic forcing. In this study we have conducted a rigorous, multi-variate detection and attribution analysis with a variety of climate, regional and hydrological models to determine the causes of the recent changes in the western snowpack, large river flow and surface air temperature. Natural variability of the western climate system has been ruled out as the major cause of the changes. Similarly, changes in solar variability and volcanic impacts are also ruled out by the analysis. We find that the observed changes in the hydrological components mentioned above can be explained well by anthropogenic forcing (green house gases and aerosols) alone. This is the first time, to our knowledge, that regional changes in the hydrological cycle have been attributed rigorously to human induced forcing. Further, since the climate models used here simulated the last 50 years of observations well, we might do well to place stock in model predictions of future changes in the West. The current simulations suggest that within the next few decades, the western United States will face a ‘climate crisis' wherein its sustainability will be severely stressed.
GC24A-04
Detection of Human Influence on Arctic Precipitation
The Arctic and the northern sub-polar regions are critical areas for climate change. The fluctuation of regional fresh water inflow to the Arctic Ocean modulates the deep ocean circulation and thus exerts a strong influence on global climate. By comparing observations to simulations from 22 coupled climate models, we find anthropogenic influence in the space-time pattern of precipitation change over the northern high-latitude (north of 55 deg. N) land area during the second half of the 20th century. The human induced Arctic precipitation change is consistent with observed increases in Arctic river discharge and freshening of Arctic water masses. This result, which is insensitive to the Arctic Oscillation influence on the observed precipitation change, provides new evidence that human activity has contributed to Arctic hydrological change.
GC24A-05
Consistency of Observed Northern European Precipitation and Temperature Trends With Regional Climate Change Projections
Often it is claimed that recent changes in northern European climate are at least partly anthropogenic even though a human influence has not yet been successfully detected. Increasing variability with decreasing scale of aggregation (and thus in general decreasing signal-to-noise ratios) as well as limitations in the ability of present day climate models in simulating the regional climate are main factors complicating successful detection and attribution at the regional scale. Hence, we choose a different approach and investigate whether the recent changes are consistent with regional climate change projections. In this study, we use gridded seasonal precipitation totals and seasonal mean temperature in the Baltic catchment, which are compared with anthropogenic climate change signals, derived from a set of different regional climate model simulations. The observed patterns of change for precipitation in winter (DJF) and spring (MAM) are consistent with model expectations. However, the model projections generally underestimate the recent change in precipitation. In summer (JJA) and fall (SON) we do not find consistency between the observed and expected patterns of change. In contrast, the patterns of change in temperature are found to be consistent with model expectations in all seasons, and the magnitude of the change is comparable as well. European trends contain large NAO-related signals, of which a major unknown part may be unrelated to the anthropogenic signal. Therefore, we also examine the consistency of recent and projected changes after subtracting the NAO signal in both the observations and in the projections. The results for precipitation in winter and spring are robust to the removal of the NAO signal, whereas considerably lower pattern similarity is found for temperature in the respective seasons. Furthermore, after removing the NAO signal from the data, trends in winter and spring temperature are considerably reduced and thus lower than the respective anthropogenic change estimates.
GC24A-06
North Atlantic warming: Fingerprints of climate change and long-term variability
Climate change in the North Atlantic Ocean has wide-spread implications for Europe, Africa, and the Americas. This study demonstrates that recent warming over the North Atlantic is linked to both long-term (including anthropogenic) climate change and multidecadal variability (~50-80 years). The multidecadal variability has basin-scale sea surface temperature anomalies accounting for ~60 percent of North Atlantic warming since 1970. In contrast, the overall long-term warming trend exhibits a pattern of cooling in regions associated with major northward heat transports, consistent with a slowdown of the North Atlantic circulation. This localized cooling has been masked in recent decades by warming during the positive phase of multidecadal variability. The next cold phase could induce a cooler North Atlantic, having serious implications for climate over Europe.
GC24A-07
Detection and Attribution of Human Influence on Climate at Sub-regional Scale: the Case of France
Human influence - mainly greenhouse gases and tropospheric sulphate aerosols -on climate has been detected on a wide range of climate variables such as surface air temperature, precipitation and sea level pressure. Global-scale detection and attribution studies have been extended to continental-scale for regions varying in size from a few to several thousand kilometres, using similar models as those used in global analyses. Here we use a specific regional framework to assess the extent to which human-induced changes in greenhouse gases and sulphate aerosols may be detected and attributed down to spatial scales of a few hundred kilometres. Using a high-quality dataset of observed temperature and large ensembles of high- resolution atmospheric model simulations, we detect a significant human influence on the 20th century evolution of summer night and day time temperature over France. The origin of the spatial warming pattern can be traced back to the spatial distribution of soil moisture climatology and a higher sensitivity of evapotranspiration to soil moisture changes in dry areas. These findings emphasize the importance of soil moisture-temperature feedbacks in influencing the future summer climate change in France and Western Europe. Our regional analysis also suggests that the France accelerated warming of the 20th century last decades is mainly due to human influence with an additional contribution from natural oceanic forcing associated to Atlantic multidecadal variability.