Climatic changes in the last 200 years (Ch. 17 & 18) 1. Is it warming? --climate proxy info (recap) -- info from historical & instrumental records 2. What.
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Climatic changes in the last 200 years (Ch. 17 & 18) 1. Is it warming? --climate proxy info (recap) -- info from historical & instrumental records 2. What does it mean?
Average northern hemisphere surface temperatures 1940-1970 cool period within warmer century 1900s unusually warm Long-term gradual T decrease Late 1400s & 1600s unusually cold 1. Is it warming? Climate proxy info recap (ice cores, tree rings, corals )
Is it warming? Info from historical & instrumental records A. direct T measurements B. glacier advances & retreats C. sea level D. cloud cover E. snow & ice cover
-- cities have low albedo (e.g. asphalt), should absorb more solar radiation -- most temperature stations located in urban areas -- this could bias temperature data -- satellite data earlier said to disagree with ground measurements, but satellite data have their own problems -- have to correct for biases
Subsurface temperatures -- Land -- measure T in boreholes, 10s of m deep -- hundreds of stations on all continents -- records extend over last 100 years -- complicated by snow, forests -- T up on all continents, by ~0.3-0.8 o C
Borehole ground temperatures Borehole locations > Models, > based on borehole data (T up ~0.3-0.8 o C)
Subsurface temperatures -- Ocean -- measure T in uppermost 3 km -- below wind mixing zone of 100 m -- records extend over last 50 years -- T up in oceans, by ~0.06 o C -- seems small, but recall large heat capacity of water (translates to a lot of energy stored in oceans– more than in atmosphere)
+0.3 -0.3 Overall T increase last 100 years Linear approx: +0.6 o C/100 years Best estimate: +0.7 o C/100 years 1A. Global average surface temperature based on direct measurements-- corrects for urban heat island effect
1B. Glacier retreat Glaciers are melting 1900-2000
1C. Two estimates of global sea level Takes into account bedrock movement & water impoundment in dams Rise caused by melting ice & thermal expansion of ocean 15 cm rise in last 100 years
1D. Cloud cover in two areas More evaporation from surface? More seed particles in air? Cooler at high-altitude? (Colder air can’t hold as much water before condensation occurs) We see more cloudiness
1E. Snow cover Since 1980 we see gradually less arctic snow cover Northern hemisphere
1E. Arctic sea ice cover Since 1970 we see 6% less area of Arctic sea ice Sea ice has also thinned from 3 to 2 m (40% decrease)
Historical & instrumental climate data: conclusions for last 100 years ____________________________________ 1. Have good evidence for overall warming 2. Evidence: direct T increase retreat of glaciers rise of sea level less arctic snow & ice 3. It seems to be more cloudy -- probably result of more surface evaporation 4. Direct T measurements imply avg. +0.7 o C/100 year
Is it warming? Yes, +0.7 o C / 100 years Direct measurement Cimate proxy data (ice cores, tree rings, corals)
2. What does recent warming mean? It depends… Can we attribute all of the warming to an anthropogenic effect?
We have large (probably anthropogenic) increase in CO 2 and CH 4 People might be contributing to an Enhanced Greenhouse Effect
2.5 / 150 = 0.017 ~1.7% increase in strength of greenhouse effect estimate Direct addition of greenhouse gases by people
Strength of Enhanced Greenhouse Effect & other factors + 2.5 W/m 2 Ozone- a greenhouse gas, but unclear if it contributes to global warming
Greenhouse debate Are people causing climate change? Probably some. How much warming is natural, how much anthropogenic? Uncertain!
Consider natural variations of climate change: recent trends A. tectonic scale B. orbital scale C. millenial D. what about solar output variations? 2. What does recent warming mean?
2A. Natural variations of climate change: tectonic scale Greenhouse world: 100 m.y. ago
Leaf outline data: ~10 o C cooling in last 50 m.y.
O-isotope data, deep oceans: ~13 o C cooling in last 50 m.y.
Natural variations of climate change: tectonic scale -10 o C / 50 m.y. = -0.000002 o C / 100 years not important for this century (infer +0.7 o C / 100 years)
2B. Natural variations of climate change: orbital scale 41,000 year cycle 23,000 year cycle (obliquity)
First warming… melting of ice sheet 16,000 to 12,000 years ago
Natural variations of climate change: solar output 2001 Text: maybe 0.1-0.2 o C / 100 yrs solar warming 2008 Text: estimate 0.07 o C/ 125 yrs = 0.09 o C/100 yrs could be significant, but can’t explain all of the recent warming (need +0.7 o C / 100 years)
An important role for solar forcing? “Some scientists argue that solar forcing explains much of the observed temperature variations in the 20th Century… The problem with this argument is that it demands strong feedbacks within the climate system to make Earth highly sensitive to small changes in the Sun’s strength, yet it has to deny the strong action of these same feedbacks… to changes in the concentration of greenhouse gases”.
An important role for solar forcing? “A more consistent interpretation is that the climate responds with similar sensitivity both to solar forcing and to changes in greenhouse gases, both of which have played roles in temperature changes during the Twentienth Century.”
So what about solar output variations? GCM models: +0.06 o C / 100 yrs to +0.3 o C / 100 yrs inferred: 0.7 o C / 100 yrs