Global Warming and the Climate System ex/2006/03/oceans_index.html

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. Understand processes that control TEMPERATURE changes Changes in temperature are associated with Climate Change

CO 2 Temperature 70 ppm A paleo perspective

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 CO 2 Methane CH 4 Nitrous oxide N 2 O Chlorofluorocarbons Water vapor H 2 O (this is the most important one, by far!)

The greenhouse effect occurs naturally... A runaway greenhouse: On Venus, the atmosphere is nearly all carbon dioxide, and the temperature is about 400°C. A moderate greenhouse: On Earth, the atmosphere contains about 360 parts per million CO 2, and the surface temperature is about 16°C. Without greenhouse gases, the Earth would be a frozen snowball!

CO 2 Temperature 70 ppm A paleo prospective 5 C

Changes in atmospheric CO 2 induced by humans Mainly, we burn fossil fuels - coal, oil, and gas. 80% of the CO 2 increase 20% of the CO 2 increase Secondarily, we cut down forests (particularly in the tropics)

CO 2 changes in the last 300 yr 70 ppm Industrial Revolution

0.8 C from thermometers from paleo records

Why doesn’t a 70 ppm change in CO 2 correspond to a 4 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 are 80% gone since early 1900’s All glaciers in tropics are melting rapidly Impacts: water supply, power generation, tourism, local climate and ecology Evidence of Global Warming in the Climate System:

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 feedback! (animation) Reduces albedo (reflectivity) of earth, allowing more radiation to be absorbed Warming temperature –Melts permafrost –Ice sheet stability? Key sea level question! Rate of warming 8x faster in last 20 years than in last

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.

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) 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

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 CO 2 represents one of the major contributors

What will happen in the future?

70 ppm

TEMPERATURE CO C

Warmer temperatures… Prediction: Doubled CO2 will warm the planet by °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?

constant emissions stop emitting and fix the concentration

Who is responsible for CO 2 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.

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 CO tons/yr CO 2 !!

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) 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)

What can or should be done about global warming? Potential for dramatic irreversible consequences Evidence that changes are beginning But there is uncertainty about details of future change 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?

Thermohaline circulation Responable for heat transport

One possibility: Predicted cooling of Europe directly related to thermohaline collapse. 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

Modern vegetation ranges Predicted vegetation ranges for future climate Plant and animal species are best adapted to certain climate conditions. Changing where plants and animals can live General pattern: northward migration of ecosystems Also some areas get wetter (shrinking grasslands) Extinctions likely as new stresses are added to threatened species

Even relatively simple things are hard to predict accurately. Best guess is that the globe will warm by about 3°C, but… Climate surprises… we don’t know enough about climate to predict it perfectly Will El Niño’s become more frequent? Will another Dust Bowl-scale drought occur? Will the ocean’s thermohaline circulation collapse? Will the west Antarctic or Greenland ice sheets melt? (125,000 years ago - melting ice = sea level 5 m above present)