Earth’s Energy Balance • A system is in energy balance (equilibrium) when incoming energy equals outgoing energy. A system in balance does not get colder or hotter on average. • Earth receives energy as radiation from the sun (shortwave). • Earth emits energy also as radiation: reflected shortwave and longwave radiation. • Changes in either energy input or output can lead to changes in earth temperature, thus climate.
Stefan Boltzmann Law All heated bodies emit energy proportional to the 4th power of absolute temperature. Hot bodies (sun) high energy short wavelengths Colder bodies (earth) lower energy longer wavelengths
31% of incoming shortwave radiation is back scattered. 49% is absorbed by the earth surface. 20% is absorbed in the atmosphere. 69% escapes as longwave radiation. 31% +69% = 100% Earth is approximately in balance. If more energy is absorbed, to remain in balance, earth must increase longwave emissions, by increasing temperature. From Chapin et al 2001
Humans have interfered with earth’s energy balance in principally two ways: Increasing the absorption of longwave radiation by accumulation of greenhouse gases in the atmosphere. Increasing the reflective properties of land cover by turning dark forest into cropland.
Albedo: The shortwave reflectance of a surface. • Oceans and lakes 0.03-0.10 • Sea ice 0.30-0.45 • Fresh snow 0.75-0.95 • Tundra 0.15-0.20 • Conifer forest 0.09-0.15 • Broadleaf forest 0.15-0.20 • Desert 0.20-0.45
Greenhouse Gases • Gases in the atmosphere that absorb longwave radiation, including CO2, H2O, methane, ozone and others. • Greenhouse gases are essential for life: without atmospheric interception of backradiation, the planet would be much colder (-18 °C). • However, small changes in the concentration of greenhouse gases an have large effects on earth temperature.
Radiative forcing: A measure (in units of energy) of change in earth’s radiation budget. A positive number implies a change towards greater energy absorption and higher temperature. A negative number implies the opposite.
Greenhouse gases compared by radiative forcing * Dichlorodifluoromethane (“freon”)
CO2 emission per person per year (in carbon equivalents) USA: 5.7 t France: 2.0 t Average non-Annex 1 countries: 0.4 t (7% of US average)
William Ruddiman, a climate scientist at the University of Virginia in Charlottesville, argues that the next ice age should have started 8000 ago, but didn’t because of the farming activities of early farmer in Europe, India and China, which released CO2 and methane into the atmosphere. New Scientist 2005
Climate prediction is complex because of : FEEDBACKS A feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedbacks are amplifying: Negative feedbacks are dampening. Quantity 1 + + Quantity 2 Quantity 1 - - Quantity 2
Negative feedbacks in the earth-atmosphere system: • Ocean solubility pump: more CO2 dissolves in water at higher CO2 concentrations in air • Carbon fertilization effects: higher CO2 levels accelerate photosynthesis , drawing more CO2 out of the air, portions of it are sequestered in oceans and on land
Positive feedbacks in the earth-atmosphere system: • Thawing permaforst in the artic releases methane, a greenhouse gas • Warming peatlands release CO2 • Shrinking sea ice and glacial cover reflects less solar radiation (albedo = reflectance of solar radiation) • Ocean warming reduces solubility of CO2 • Warmer temperatures on water and land increases water evaporation, increasing air humidity and the greenhouse effect • The effect of increasing cloud cover is unclear
So is it getting warmer? 2005 AND 2010 TIED FOR WARMEST YEAR ON RECORD. Before that the warmest year was 1998. 2002 and 2003 were the second and third warmest years, respectively 2004 was the forth warmest year on record. .
Climate Prediction • Done by global climate models, there are several (8) using different approaches and parameters, and arriving at different conclusions. • Tested by “hindcasting” – how good they are predicting past climate that we know. • The IPCC* weighs the prediction of these models according to their ability to hindcast, putting together the most highly supported scenario • *IPCC: Intergovernmental Panel of Climate Change • An international body for the assessment of climate change • Established by the United Nations Environment program and the World Meteorological Organization in 1988 • Produce assessment reports every 5 years. • Reports have been puublished in 1990, 1995, 2001, 2007. • The organization was honored with the Nobel Peace Price in 2007.
Climate predictions depend strongly on “scenarios” = assumptions about political response to climate change The 2001 IPCC Report had 3 scenarios: “Business as usual” prediction: Global mean temperature could rise by 4° C by 2100.
The 2007 IPCC Report had more complex scenarios: A1 story line: rapid economic growth, population peaks in mid-century, rapid development of new and more efficient technologies, increased global equity. A2 story line: heterogeneous world, nations progress at their own pace, global population keeps growing. B1 story line: convergent world as in A1, but with transformation into a service and information economy and less material intensity. B2 story line: focus on local solutions to social economic and environmental sustainability.
Worst scenario: A1FI : rapid economic growth, globalization, fossil fuel intensive development Best scenario: B1: sustainable and equitable growth, globalization
Model predictions vary, but they all predict very significant global temperature rise (worst case, A1FI scenario).
Since 1979, the size of the summer polar ice cap has shrunk more than 20 percent. (Illustration from NASA)
What could happen in the next 100 years (rising sea levels):
Anticipated effects of unchecked climate change by 2100: • Melting of glaciers, polar caps and and sea ice. • Global sea level rise by as much as 6 m. • Redistribution of global climate zones, especially at high latitudes) • More extreme weather: longer droughts, more violent storms (flooding) • Massive population migrations and increases in civil war activities.