Daniel Rus
Complex Dynamic Systems
November 3, 2007
IPCC Forth Assessment Report
This report describes the progress in the understanding of the human and natural causes of climate change as well as the observed climate change, climate processes and attribution, and predictions of future climate change. Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties cause alterations to the energy balance of the climate system. These changes are expressed in terms of radiative forcing.
Global atmospheric concentration of carbon dioxide, methane and nitrous oxide have increased greatly by human activity since 1750. Concentrations of greenhouse gases now far exceed pre-industrial values determined from ice cores spanning thousands of years. The increase in carbon dioxide is primarily due to the use of fossil fuel and land use change. Carbon dioxide is the most important anthropogenic greenhouse gas. The global atmospheric concentration has increased greatly since pre-industrial values from about 280ppm (parts per million) to 379ppm in 2005. In 2005, the atmospheric concentration of carbon dioxide greatly exceeded the natural range of 180 to 300ppm over the last 650,000 years. The primary source of the increase in atmospheric concentration of carbon dioxide results from the use of fossil fuels, with land use change providing another significant but smaller contribution. The annual fossil carbon dioxide emissions have increased from an average of 6.4 [6.0 to 6.8]GtC per year in the 1990s to 7.2 [6.9 to 7.5]GtC per year in 2000-2005.
The increase of methane and nitrous oxide are due primarily due to agriculture. The concentrations of methane have increased from a pre-industrial value of about 715ppb (parts per billion) to 1732ppb in the 1990s. The methane concentration in 2005 was about 1774. This concentration greatly exceeds the natural range of 320 to 790 ppb of the last 650,000 years. The growth rates of methane have declined since 1990 and has stayed consistent with total emissions. The global atmospheric nitrous oxide concentration has increased from 270ppb during pre-industrial times to 319ppb in 2005. The combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is +2.3 [+2.07 to +2.53] Wm m-2. The global average net effect of human activities since 1750 has been one of warming, with the radiative forcing of +1.6 [+0.6 to +2.4] W m-2 (exponent).
Aerosols such as sulphate, organic carbon, black carbon, nitrate, and dust have produced a cooling affect. The total direct radiative forcing of these is -0.5 [-0.9 to -0.1] W m-2 and have caused an indirect cloud albedo forcing of -0.7 [-1.8 to -0.3] W m-2
Several other sources contribute to the overall radiative forcing. Tropospheric ozone changes caused by the emission of ozone forming chemicals such as nitrogen oxides, carbon monoxide, and hydrocarbons contribute +.35 [+0.25 to +0.65] W m-2. Changes in halocarbons contribute +0.34 [+0.31 to +0.37] W m-2.
Warming of the climate system has caused increases in the global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. This is now evident from observations. Eleven of the the last twelve years (1995-2006) are ranked as the 12 warmest years since 1850. The total temperature increases from 1850-1899 to 2001-2005 is 0.76C [0.57C to 0.95C]. The average atmospheric water vapour content has increased since the 1980s over land and ocean as well as in the upper troposphere. The increase is broadly consistent with the extra water vapour that warmer air can hold. The average temperature of the global ocean has increased to depths of at least 3000m. The ocean has been absorbing more than 80% of the heat added to the climate system. These affects cause the ocean to expand, and thus contributing to sea level rise. The decline in the average of mountain glaciers and snow cover in both hemispheres with the decrease in glaciers and ice caps have contributed to sea level rise. New data suggests that that losses from the ice sheets of Greenland and Antarctica have contributed to sea level rise. The average global sea level rise through 1961 to 2003 was 1.8 [1.3 to 2.3] mm per year, but this rate grew faster over 1993 to 2003 to about 3.1 [2.4 to 3.8] mm per year.
Numerous long term changes in climate have been observed. These include changes in arctic temperatures and ice, changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones. Average arctic temperatures increased at almost twice the global average rate in the past 100 years. Satellite data has shown that since 1978 the annual average arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3]% per decade. There is a greater decrease during the summer of 7.4 [5.0 to 9.8]% per decade. The temperatures at the top of the permafrost layer in the Arctic have increased since the 1980s has by up to 3C. There has been a decrease in maximum area covered by seasonally frozen ground by about 7%.
Precipitation has increased significantly in eastern parts of North and South America, northern Europe and northern and central Asia. Changes in precipitation and evaporation over the oceans are suggested by freshening of mid and high latitude waters together with increased salinity in low latitude waters. Mid latitude westerly winds have strengthened in both hemispheres since the 1960s. Droughts have become longer and more intense and have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Droughts have been linked to higher temperatures and precipitation, as well as changes in sea surface temperatures, wind patterns and decreased snow cover. The frequency of heavy precipitation has stayed consistent with warming and increases of atmospheric water vapor and has increased over most land areas. Over the last 50 years widespread changes in extreme temperatures have been observed. Hot days, hot nights, and heat waves have become more frequent, while cold days, cold nights, and frost have become less frequent.
Paleoclimatic data shows that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer for an extended period of time was about 125,000 years ago and the reduction in the polar ice volume led to 4 to 6 m of sea level rise. The average Northern Hemispheres temperature during the second half of the 20th century were higher than during any other 50-year period in the last 500 years. It is possibly the highest in at least the past 1,300 years.
Human influences have extended to other aspects of climate such as ocean warming, continental average temperatures, temperature extremes and wind patterns. Climate change of the last 50 years cannot be explained without external forcing and is not due to natural causes alone. There has been significant anthropogenic warming over the past 50 years over each continent except Antarctica and show greater warming over land than over the ocean, and their changes over time, are only simulated by models that include anthropogenic forcing. Anthropogenic forcing has contributed to changes in wind patterns. This affects extra-tropical storm tracks and temperature patterns in both the North and South Hemispheres.
Even if all concentration of greenhouse gases and aerosols are kept concentration at year 2000 levels, a warming of about 0.1C per decade would be expected primarily due to the slow response of the ocean. If greenhouse gas emissions are continued, it can be expected that current rates of climate change and warming in the 21st century would increase significantly. Increasing atmospheric carbon dioxide concentrations lead to increasing acidification of the ocean. Projections give reductions in average global surface ocean pH16 of between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times. Widespread increases in thaw depth are projected over most permafrost regions. Sea ice is projected to shrink in both the Arctic and Antarctica. Heat waves and extremely hot temperature are going to become more frequent. Future tropical cyclones will become more intense, with larger peak wind speeds and more heavy precipitation. Projections state that there will be increases to in the amount of precipitation in high latitudes, while precipitation will decrease in subtropical regions by as much as about 20%. Based on model simulations, it is believed that the meridional overturning circulation of the Atlantic ocean will slow down during the 21st century and by 2100, it will have slowed down by 25%. Even if greenhouse gas concentrations were to be stabilized, global warming and sea level rise would continue for centuries.Global temperatures would still be on the rise with further increase in global average temperature of about 0.5C, even if radiative forcing was stabilized. The loss of ice mass in Greenland is proportional to the rise of temperature. The ice mass balance will become negative in a global average warming in excess of 1.9C to 4.6C and if this continued for a millennia, it would lead to the complete elimination of the Greenland Ice Sheet. This would result in 7 m rise in the sea level.
Anthropogenic carbon dioxide emissions from both the past and the present will continue to contribute to warming, climate change, and sea level rise for more than a millennium due to the time required to remove the gases from the atmosphere.