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SCHEDULE |
Climate and Climate Change
Notes for Tuesday March 5 Here is an overview
of the science of global warming, starting with specifying emissions scenarios
for the future and ending with estimating the impacts of those emissions.
emissions ---> concentrations
---> radiative forcing
feedbacks In considering emissions, concentration and radiative forcing, it's necessary to take into account, not only CO2, but the full suite of greenhouse gases and aerosols. Here's a check list for all the things that need to be estimated. Emissions Concentrations Radiative Forcing carbon dioxide
Future carbon dioxide concentrations will rise, even if emissions stay constant. The future rate of rise of carbon dioxide will depend upon the uptake by the land and the ocean. The observed rate of increase is roughly half the rate at which CO2 is being added to the atmosphere by the burning of fossil fuels, but that feacrion varies from one year to the next. http://www.ipcc.ch/pub/tar/wg1/fig3-3.htm There's evidence
that the uptake by the land was greater during the1990's than during the
1980's and the rate of increase of atmospheric CO2 did not keep up with
the rising rate of emissions. Here are the numbers based on analysis
of CO2 and O2 measurements.
BURNING = ATM INCR + OCEAN UPTAKE + LAND UPTAKE estimates
in Gt / year
198O's
1990's
Deforestation in the tropics is estimated to have released more CO2 into the atmosphere than reforestation at higher latitudes has removed. Here are the numbers. The land uptake is different from the value given above because it was estimated based on different data. LAND UPTAKE = LAND USE CHANGE - RESIDUAL SINK 1989-98
0.7
=
-1.6
+2.3
At this point we do not know where the missing carbon is going or how this partitioning is likely to evolve in the 21st century. For example, will the oceans continue to take up as much as they have in the past or will they become 'saturated'? Will the mysterious 'land sink' continue to be operative? A similar analysis need to be made for each of the constituents listed in the table. For shorter lived constituents, concentration tends to be proportional to emissions.
Radiative Forcing carbon dioxide
Estimates of the radiative forcing by the above constituents are given in the IPCC Report http://www.ipcc.ch/pub/tar/wg1/figts-9.htm The warming induced by the combined radiative forcing shown in this figure amounts to ~1 C for a doubling of CO2. The actual projected warming is higher than that, by a factor of 2-3. The difference is due to the existence of feedbacks.
Feedbacks inferred from simple calculations, models water vapor
+ constant relative humidity yields a doubling;
contested
Climate change (planetary):
inferred from global climate models
Sea level rise:
inferred from ocean models
Principally due to the expansion of sea water as it warms. Melting sea ice doesn't contribute, since sea ice is already floating. Melting of mountain glaciers and continental ice sheets is ignored.
Climate change (regional):
inferred from regional climate models
Ecological impacts
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