Climate Change and human Migration

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Evidence of the effect of external influences on the climate system has continued to accumulate since the TAR. There is widespread evidence of anthropogenic warming of the climate system in temperature observations taken at the surface, in the free atmospheric and in the oceans. It is very likely that greenhouse gas forcing has been the dominant cause of the observed global warming over the last 50 years, and it is likely that greenhouse gases alone would have caused more warming than has been observed during this period, with some warming offset by cooling from natural and other anthropogenic factors. It is highly likely (>95%) that warming during the past half century cannot be explained without external forcing, or based on known natural external causes alone. The warming occurred in both oceans and atmosphere, and took place at a time when non-anthropogenic external forcing factors would likely have produced cooling. Studies that attribute warming to greenhouse gas forcing rely on a range of observational data sets, variables, climate models and methods. Anthropogenic warming has now been detected on continental and sub-continental scales in a range of studies, and evidence is emerging that surface temperature extremes have likely been affected. In addition, anthropogenic influence is becoming apparent in other parts of the climate system. While many uncertainties remain, the physical consistency of the many different lines of evidence increases our confidence in the conclusion that anthropogenic forcing has caused substantial change in the climate system over the past century. Human-induced warming of the climate system is widespread. Anthropogenic warming of the climate system can be detected in temperature observations taken at the surface, in the free atmosphere and in the oceans. Multi-signal detection and attribution analyses, which quantify the contributions of different natural and anthropogenic forcings to observed changes, show that greenhouse gas forcing during the past half century would likely have resulted in greater warming than observed if there had not been an offsetting cooling effect from aerosol and other forcings. This conclusion takes into account observational and forcing uncertainty, and is robust to the use of different climate models, different methods for estimating the responses to external forcing, and variations in the analysis technique. Further evidence has accumulated of an anthropogenic influence on the temperature of the free atmosphere since widespread radiosonde measurements started in the late 1950s. The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the influence of anthropogenic forcing, particularly greenhouse gases and stratospheric ozone depletion. The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause. It is likely that anthropogenic forcing has contributed to the observed warming of the upper several hundred meters of the ocean during the latter half of the 20th century. Anthropogenic forcing, resulting in thermal expansion from ocean warming and glacier and ice sheet melt, is likely the largest contributor to sea level rise during the latter half 20th century. Anthropogenic influence is now detectable in regional surface temperatures. The anthropogenic signal in surface temperature changes has now likely been detected in all inhabited continents and many subcontinental land areas. The ability of models to simulate the temperature evolution on these scales and the detection of anthropogenic effects on individual continents provides stronger evidence of human influence on the global climate than was available to the TAR. Evidence of anthropogenic influence on surface temperature extremes is emerging. Surface temperature extremes have likely been affected by anthropogenic forcing. Many indicators of impact-relevant surface temperatures, including the annual numbers of frost days, warm days and cold days, show changes consistent with warming. Anthropogenic influence has been detected in some of these indices, and there is evidence that anthropogenic forcing may have substantially increased the risk of extremely warm summer conditions regionally, such as the 2003 European heat wave. Anthropogenic influence is becoming apparent in other parts of the climate system. Anthropogenic forcing has likely contributed to recent decreases in Arctic sea ice extent. There is evidence of a decreasing trend in global snow cover and widespread retreat of glaciers, consistent with warming, and evidence that this melting has also likely contributed to sea-level rise. Trends in the Northern and Southern Annular Modes over recent decades, which correspond to sea level pressure reductions over the poles, are likely related in part to human activity. Models reproduce the sign of the Northern Annular Mode trend, but the simulated response is smaller than observed. Models including Do Not Cite or Quote 9-3 Total pages: 98 Second-Order Draft Chapter 9 IPCC WG1 Fourth Assessment Report both greenhouse gas and stratospheric ozone changes simulate a realistic trend in the Southern Annular Mode, leading to a detectable human influence on global sea level pressure. The response to volcanic forcing simulated by some models is detectable in global annual mean land precipitation during the latter half of the 20th century. Observed, large scale changes in land precipitation over the 20th century appear to be qualitatively consistent simulations of the 20th century, suggestive of a possible human influence. Observed increases in heavy precipitation appear to be consistent with increases that are expected to occur with warming, although it is unclear whether these changes are distinguishable from natural variability. The observed increase in the proportion of very intense hurricanes is the same direction but not magnitude as suggested by theoretical studies and modelling stuidies of projected 21st century change. Inadequacies in process knowledge and understanding of natural variability, modelling, and monitoring of tropical cyclones inhibit attribution to anthropogenic factors. Proxy climate data have been used to increase confidence in models and the role of external influences on climate. Coupled climate models used to predict future climate have been used to understand past climatic conditions such as those of the Last Glacial Maximum and the Mid-Holocene. While many aspects of these past climates are still uncertain, climate models are successful in reproducing their broad features when forced with boundary conditions and radiative forcing for those periods. A large fraction of Northern Hemisphere interdecadal temperature variability in reconstructions of the past 7 centuries is very likely attributable to natural external forcing. Furthermore, the 20th century warming evident in these records, most of which end by the mid-20th century, is likely due in part to anthropogenic forcing. Estimates of the climate sensitivity are now better constrained by observations. Estimates of the equilibrium climate sensitivity and the transient climate response, their uncertainties and the ability of observable quantities to constrain them are now better understood. The equilibrium climate sensitivity is likely 2 to 4.5°C, with a most likely value of approximately 3°C, although the upper limit remains difficult to constrain from observations. The transient climate response is better constrained and is unlikely to be greater than 2.8°C at the time of CO 2 doubling in response to a 1% per year increase in CO 2. Overall consistency of evidence. Although uncertainties remain, many observed changes in the climate over the 20th century are distinct from internal variability and consistent with the expected response to anthropogenic forcing. This consistency is apparent in many observations, including surface and free atmospheric temperature, ocean temperature and sea-ice extent, some large scale features of the atmospheric circulation. The simultaneous increase in energy content of all the major components of the climate system supports the conclusion that the cause of the warming is highly unlikely (<5%) to be the result of internal processes. Qualitative consistency is also apparent in some other observations, including snow cover, glacier retreat, and heavy precipitation. Remaining uncertainties. Further improved models and analysis techniques have increased confidence in the understanding of the influence of external forcing on climate since the TAR. However, estimates of some radiative forcings remain uncertain, including aerosol forcing and inter-decadal variations in solar forcing. However, the robustness of surface temperature attribution results to forcing and response uncertainty has been evaluated with a range of models, forcing representations and analysis procedures. The potential impact of the remaining uncertainties has been considered, to the extent possible, in the overall assessment of every line of evidence listed above.. There is less confidence in the understanding of forced changes in other variables, such as surface pressure and precipitation. The extent to which uncertainties in observations, forcing and model formulation contribute to a lack of quantitative agreement is not presently understood. The detection and attribution of external influences is performed by comparing observed changes against those that could plausibly arise from natural internal variability. Improvements in models and instrumental and proxy climate records have increased confidence in climate model simulated internal variability. However, uncertainty remains because the available observational records are influenced by external forcing, are often too short to provide accurate estimates of decadal and longer time scale variability, and some have inadequate spatial coverage and accuracy. While reduced,...

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