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

2. 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.

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

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

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

6. 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.

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

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

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

10. 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

11. 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

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

13. 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)

14. 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,
Observed signal larger than model variability.
Temperature anomoly (oC)
Stouffer et al, 1999

15. 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

16. 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

17. 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.

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

19. 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?

20. Extra slides

21. IPCC 2001

22. IPCC 2001