Global Warming and the Climate System
Kim Cobb EAS http://shadow.eas.gatech.edu/~kcobb
To understand how climate has changed in the past, we need to use records of climate preserved in ice cores, ancient tree rings, coral bands, and other “paleoclimatic” sources:
Temperature is a state variable of the Climate System. Changes in temperature are associated with Climate Change
Understand processes that control TEMPERATURE changes
A paleo perspective CO2
range = 200 to 280ppm
80 ppm
Temperature
range = 5ºC
5ºC
What’s the greenhouse effect?
greenhouse gases in the atmosphere trap heat at the Earth’s surface and prevent it from escaping.
These gases include: •Carbon dioxide CO2 •Methane CH4 •Nitrous oxide N2O •Chlorofluorocarbons •Water vapor H2O
(this is the most important one, by far!)
The greenhouse effect occurs naturally...
A moderate greenhouse: On Earth, the atmosphere contains about 360 parts per million CO2, and the surface temperature is about 16°C. Without greenhouse gases, the Earth would be a frozen snowball!
A runaway greenhouse: On Venus, the atmosphere is nearly all carbon dioxide, and the temperature is about 400°C.
Changes in atmospheric CO2 induced by humans
Mainly, we burn fossil fuels - coal, oil, and gas. 80% of the CO2 increase Secondarily, we cut down forests (particularly in the tropics) 20% of the CO2 increase
CO2 changes in the last 300 yr
380
100 ppm
2003 2004 2005 2006
2050
379 381 383 385
ppm ppm ppm ppm
473 ppm 573 ppm
Industrial Revolution
2100
Theory predicts that increasing CO2 will warm the planet – what do we see?
from thermometers
from paleo records
1ºC
But why doesn’t an 80
ppm change in CO2 correspond to a 5 C change?
The climate system does not reach equilibrium instantaneously Other processes can change the equilibrium temperature.
Other greenhouse gases are also increasing
Evidence of Global Warming in the Climate System
Glacier melting
• Kilimanjaro: ice caps have melted back by 80% since early 1900’s; will be gone by 2020 • All glaciers in tropics are melting rapidly • Impacts: water supply, power generation, tourism, local climate and ecology
Arctic - the most sensitive ecosystem?
• Arctic sea ice has shrunk by 1 million sq km and thinned from 3.1m to 1.8m average
– More freshwater, reduced ability to travel over ice – All summer ice gone in this century
• Ecological consequences huge!
Arctic warming
• Sea ice melting
– Key ALBEDO feedback! – Reduces albedo (reflectivity) of earth, allowing more radiation to be absorbed
http://www.gsfc.nasa.gov/topstory/2003/1023esuice.html
• Warming temperature
– Melts permafrost – Ice sheet stability? Key sea level question!
• Rate of warming 8x faster in last 20 years than in last 100
1979
2003
Arctic ecosystems impacts
• Reduced ice: less algal production under ice = undermines base of food chain
• Seal pups emerge just when ice is melting - earlier melt means they are exposed before ready to thrive • Caribou need ice to island-hop; they are falling through as ice thins • Polar bears hunt on ice in winter, retreat to land in summer. Less ice forces them onto land earlier
More intense storms…
Hurricanes get their energy and staying power from warm water in the tropical oceans. As waters get warmer, we expect that hurricanes will become more intense.
Increase in Category 4 & 5 hurricanes observed. Katrina does not prove this!
A weak 2006 season does not disprove this!
Higher sea level…
• As water warms, it expands (“thermal expansion”).
• Glaciers are melting • Observation: 3mm/yr in past few decades • Prediction: ~0.5m rise by the end of this century, 2-4m in 500 years
This will have a major impact on •Developed coastal regions •Low-lying island nations
•Intensity of coastal flooding during storm surges
•Coastal ecosystems (e.g. mangroves, estuaries)
What is the role of human emissions?
Are we responsible or is it just a natural cycle of the climate system?
The temperature in the climate system is controlled by many different processes,
however anthropogenic forcing by CO2 represents one of the major contributors
What will happen in the future?
70 ppm
CO2
TEMPERATURE
4.5 C
Warmer temperatures…
Prediction: Doubled CO2 will warm the planet by 1.8-5.8°C (before 2100) Some areas warm up more than others. Continents warm faster than oceans. Higher latitudes warm more than low.
Map of predicted temperature change for a doubling of atmospheric CO2
Can we do anything about it?
Who is responsible for CO2 emissions?
(these data are relatively correct even for today)
• Mainly the U.S.: 25% • Other industrialized countries: 25% • Developing world: 30% • Remaining 20% split: Mideast, former Soviet Union, E. Europe.
• The U.S. also has the largest per capita carbon emissions
Personal choices make a difference!
• About one third of all GHG emissions in the US are related to personal choices
– How you travel – Home energy use – Waste production
• The remaining 2/3 is related to factors like industry, agriculture, and business uses
3.1 tons/yr CO2
14.3 tons/yr CO2!!
Is human-caused global warming for real?
The evidence
• • • • • Greenhouse gases are rising due to human activity The “greenhouse effect” is real (past climate example) The Earth is warming up No other obvious causes are seen This point is no longer debated by climate scientists (IPCC)
http://www.ipcc.ch/ What can be done?
• Personal choices • Political actions (e.g. a successor to the Kyoto treaty capping carbon emissions) • Industry initiatives (e.g. hybrid gas-electric cars)
• Potential for dramatic irreversible consequences • Evidence that changes are beginning • But there is uncertainty about details of future change
What can or should be done about global warming?
What gets in the way of action?
• Uncertainty
– (don’t we always make decisions in the face of uncertainty?)
• Economics
– (is this really an issue? How are estimates of costs made, and what are the costs of doing nothing? What are the benefits of acting?
• Politics
– Do governments follow scientific advice?
Are more dramatic effects possible? How about the movie
The DAY after TOMORROW
FACT: Some computer models suggest that the thermohaline circulation could abruptly shut down as a result of global warming.
FICTION: .
The Thermohaline circulation today
Responsible for heat transport to North Atlantic
Slow/shut down thermohaline circulation?
• Future climate ==> more precipitation over North Atlantic • Surface ocean there will become fresher, less dense. • Leads to weaker or failed thermohaline circulation
Predicted cooling of Europe directly related to thermohaline collapse. Perhaps more complicated effects that we cannot predict, but NYC will NOT freeze over!
End of lecture
Changing where plants and animals can live
Plant and animal species are best adapted to certain climate Predicted vegetation ranges conditions. Modern vegetation ranges
for future climate
• General pattern: northward migration of ecosystems • Also some areas get wetter (shrinking grasslands) • Extinctions likely as new stresses are added to threatened species
we don’t know enough about climate to predict it perfectly
Even relatively simple things are hard to predict accurately. Best guess is that the globe will warm by about 3°C, but…
Climate surprises…
• • • •
Will Will Will Will ago
El Niño’s become more frequent? another Dust Bowl-scale drought occur? the ocean’s thermohaline circulation collapse? the west Antarctic or Greenland ice sheets melt? (125,000 years - melting ice = sea level 5 m above present)