Climate Change 2007: The Physical Science Basis

Summary of Selection 26

Carbon dioxide, methane, and nitrous oxide gases have increased in our atmosphere as a result of human activities. CO2 is the most significant anthropocentric source, and has increased greatly since pre-industrial times; from 280ppm to 379ppm (2005). This is primarily from the burning of fossil fuels, but also from land use change. Methane has increased from 715ppb (pre-indust.) to 1774ppb (2005), due to agriculture and fossil fuel use. Nitrous oxide is also way up from 270ppb to 319ppb, also mostly due to agriculture. All the pre-industrial levels were found using ice cores. The net effect of anthropogenic cooling and warming influences on the climate have a net warming effect. The warming influences include: radiative forcing due to CO2, CH4, NO2, tropospheric ozone, halocarbons, surface albedo and solar irradiance. Some cooling influences are aerosols and cloud albedo forcing.

There are many direct observations scientists have made about climate change. Some more significant ones are:
-a linear warming trend
-increased water vapour in air
-increased average ocean temperature
-decreased glaciers and snow cover
-decrease in the ice sheets of the Antarctic and Greenland
-increased global sea level
There have also been many changes observed in the Arctic. These include changes in temperature and ice levels, permafrost, precipitation, ocean salinity, wind patterns, extreme weather, heat waves and cyclones. These global and Arctic trends will all increase as global warming increases. The Atlantic Ocean meridional overturning circulation (MOC) will slow and temperature will increase in this area. It is very likely that all these are due to human activity and would not have happened on its own.

A 0.2C increase in temperature/decade is estimated from models, depending on whether current levels are kept constant, increase, or decerase (best-worst scenario estimate). All the effects (in above paragraph) will continue for centuries because of the long timescales of chemical climate processes/feedbacks, even if we stabilized greenhouse gas concentrations now. Removal of CO2 etc. from the atmosphere takes a long time so we will continue to have negative effects. There are some limitations on models due to lack of information and due to the nature of climate change, it is very unpredictable, but this is the best understanding we have currently.