The Human Influence on Climate: How much is known, What’s in store for us? Loretta Mickley Harvard University CO2 concentrations, Mauna Loa

Some background: What do I do? Develop chemical models of the atmosphere Analyze measurements of chemical species Examine the link between chemistry and climate Calculated ozone concentrations in ppb at the surface, annually averaged.

The earth’s energy budget: Incoming + sulfate aerosol Soot aerosol absorbs incoming sunlight. Sulfate aerosol reflects sunlight back to space. + soot aerosol

The earth’s energy budget: Outgoing Infrared radiation = heat Without greenhouse gases, earth would be cold (0oF) + soot aerosol

What could change the energy budget (and climate)? + aerosol + aerosol + aerosol (ice) ocean

Concentrations of carbon dioxide and methane have risen sharply since pre-industrial times. Carbon dioxide: 33% rise Methane: 100% rise BW 5 The MetOffice. Hadley Center for Climate Prediction and Research.

What is radiative forcing? Radiative forcing = calculated heating/cooling of atmosphere, convenient yardstick. Units = W/m2 Radiative forcing since 1760 IPCC 2001

How have global surface temperatures changed over the last 140 years? SPM 1a Jones et al 2001, Folland et al. 2001

Variations in surface temperature over the Northern Hemisphere for the past 1,000 years Large uncertainty Mann et al, 1998

How has climate varied over the last 25,000 years? Back in time Last 10,000 years = relatively warm period, era of agriculture, rise of civilization Data from ice cores. Proxy for temperature changes Last ice age IPCC 2001

Temperature changes going 400,000 years back Back in time Series of ice ages. Reasons for temperature swings = changes in earth’s orbit or tilt, amplified by changes in greenhouse gases?? Sowers and Bender, 1995

How has temperature changed by region? Most rapid temperature increase over high northern latitudes. (Possible reason= positive ice feedback) Jones et al, 2001

How do you diagnose reasons for observed changes? No way to do control experiment (n=1) Models = tools to understanding past climate change, predicting future. Two kinds of models: Energy balance models (simpler, quicker to run) General circulation models (calculate circulation of winds and ocean currents, more complicated)

Could the observed warming be due to random variability? Observed temperature Model output 3 attempts to simulate climate without any forcings (no volcanos or solar changes). Also, observed temperature trend, 1850-2000. Observed signal larger than model variability. Temperature anomoly (oC) Stouffer et al, 1999

What caused the warming over the last 150 years? Volcanos, solar changes Greenhouse gases, manmade aerosol 3 attempts to fit model changes to observations. Best match All forcings Stott et al, 2000, Tett et al 2000

Can we simulate the last 1000 years in Northern Hemisphere? reconstructed temperatures thermometer temperatures energy balance model Model includes volcanic aerosol, solar variations, greenhouse gases and manmade aerosol. Much “tuning” in this model! Crowley et al, 2000

Caveats regarding models Models usually calibrated to get present-day climate right. Models unable to reproduce abrupt climate change, have difficulty with patterns of changes. Back in time Series of ice ages. Models have trouble capturing large temperature swings.

How are temperatures projected to change over the next 100 years? Model result depends on which scenario is chosen. Estimated temperature increase = 1.4 - 5.8 oC (large uncertainty) IPCC, 2001

What other possible changes could we see? More heat waves? More stagnation events (and more high ozone days)? More droughts in some regions, more intense rainfall elsewhere? (changes in hydrological cycle) Many uncertainties! Possible surprises?

Extra slides

IPCC 2001

IPCC 2001