Models
• Questions?
• What about uncertainty?
– Models may have different average states
– Want to model climate change

What happened in the 20 th century and why
• Six years after the IPCC First Assessment we see that there is discernable evidence that humans have influenced climate
• Instrumental record and internal variability from models
– Very unlikely that warming is due to internal variability alone
• Natural and human forcing give the result we expect
– Without human forcing only the first half of the 20th century looks correct

External forcings
• Some forcings have distinct climate responses
• Vertical temperature response depends on the forcing
• Examples:
– Greenhouse gas forcing should result in warming in the troposphere and cooling in the stratosphere, increase in troposphere height (models and observation)
– Solar forcing results in a general warming of the atmosphere (models) weak signal
– Volcanic forcing results in cooling of the troposphere and warming in the stratosphere (modeled and observed)

Vertical structure of the atmosphere

Solar Volcano
Greenhouse Gas
All forcings

Model results with natural and human
Both human and natural forcing
Natural forcing only

Blue is natural forcing only Red is all forcings

Difficulties in looking at the small scale
• It is easier to see trends on a continental scale over more than 50 years
• Looking at short time scales and areas smaller than continents makes it more difficult distinguish forcing
• Think about model resolution and grid size.
– Puget Sound vs. The US or N.America

Change in surface temperature extremes
• Global warming connoted with change in mean temperature.
• However, change in extreme temperatures (tails of the distribution) consistent with general warming.
• In midlatitudes, manifested as:
– Reduction in:
• Frost days
• Cold nights
– Increase in:
• Warm nights
• Global and regional studies
IPCC 2007

Anthropogenic influence elsewhere…
• …hydrosphere
– Interdecadal changes
• Lots of variability (see right)
– Latitudinal changes
• Robust result from IPCC models
• Wet get wetter and dry get drier (see right)
IPCC 2007

IPCC 2007
…hydrosphere
• An example
– Drought in the Sahel
• Decrease in rainfall from 50s - 90s
• Two leading hypotheses were:
• Land use changes altered albedo.
• Different large scale circulation resulting from SST changes.
• Modeling experiments suggest that observed SSTs explain a majority of the precipitation variability
– Lends credence to latter hyp.

…cryosphere and hydrosphere

…cryosphere
• Decrease inconsistent with natural forcing alone.
• The decline and thinning appears to be largely, but not wholly, due to greenhouse gas forcing.
NSIDC 2008 IPCC 2007

…cryosphere + hydrosphere
• All that melting sea ice…no contribution to SLR.
• However, glaciers and ice caps do.
• At least half of observed SLR due to thermal expansion
• Decrease in NH snow cover extent consistent with warming. Evidence that melting has likely contributed to
SLR.
IPCC 2007

…tropical cyclones
• More likely than not that anthropogenic influence has contributed to increases in the frequency of the most intense tropical cyclones.
• Suggestion of a greater number of cyclones in the
Atlantic ocean.
Holland and Webster (2007)

Climate changes due to human activity
IPCC 2007
Virtually certain > 99% Very likely >90% Likely >66% More likely than not > 50%

Human induced GHG forcing
• Temperature changes in the atmosphere, the ocean and on the surface are widespread.
– Change is very significant compared to internal variability
– Internal variability alone could not cause the global trends we see
• Warming is detected on all continents except
Antarctica
– Little observation of Antarctica
• Second half of 20 th century was warmest in 1.3 kyr
– Consistency across different evidence supports warming is human induced