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Transcript
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?
1. Is it warming?
Climate proxy info recap
(ice cores, tree rings, corals)
Average northern hemisphere surface temperatures
Long-term gradual
T decrease
1900s unusually
warm
Late 1400s
& 1600s
unusually
cold
1940-1970 cool
period within
warmer century
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
1A. Direct temperature measurements at surface
Temperature
stations – more
in cities, northern
hemisphere
So get bias,
especially owing
to Heat Island Effect
What is the Urban Heat Island Effect?
What is the Urban Heat Island Effect?
-- 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 oC
Borehole ground temperatures
Borehole
locations
>
Models,
>
based on
borehole data
(T up ~0.3-0.8 oC)
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 oC
-- seems small, but recall large heat capacity
of water (translates to a lot of energy stored
in oceans– more than in atmosphere)
Shallow
ocean
warming
Up 0.06 oC
1A. Global average surface temperature
based on direct measurements-corrects for urban heat island effect
+0.3
Overall T increase
last 100 years
-0.3
Linear approx: +0.6 oC/100 years
Best estimate: +0.7 oC/100 years
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
We see more
cloudiness
More evaporation from
surface?
More seed particles in air?
Cooler at high-altitude? (Colder
air can’t hold as much water
before condensation occurs)
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 oC/100 year
Is it warming?
Yes, +0.7 oC / 100 years
Cimate proxy data
(ice cores, tree rings, corals)
Direct
measurement
2. What does
recent warming
mean?
It depends…
Can we attribute
all of the warming to
an anthropogenic
effect?
People might be
contributing to an
Enhanced Greenhouse
Effect
We have
large
(probably
anthropogenic)
increase in
CO2 and CH4
estimate
2.5 / 150 = 0.017
~1.7% increase
in strength
of greenhouse
effect
Direct addition of
greenhouse gases
by people
+ 2.5 W/m2
Strength of
Enhanced
Greenhouse
Effect
& other
factors
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!
2. What does recent warming mean?
Consider natural variations of
climate change: recent trends
A. tectonic scale
B. orbital scale
C. millenial
D. what about solar output variations?
2A. Natural variations of climate
change: tectonic scale
Greenhouse world: 100 m.y. ago
Leaf outline
data: ~10 oC
cooling in
last 50 m.y.
O-isotope
data, deep
oceans:
~13 oC
cooling in
last 50 m.y.
Natural variations of climate
change: tectonic scale
-10 oC / 50 m.y. = -0.000002 oC / 100 years
not important for this century
(infer +0.7 oC / 100 years)
2B. Natural variations of climate
change: orbital scale
41,000 year
cycle
(obliquity)
23,000 year
cycle
First warming…
melting of ice sheet
16,000 to 12,000 years
ago
Then cooling…
Synthesis of studies using climate
proxies (ice cores, tree rings, corals)
Average northern hemisphere surface temperatures
Long-term gradual
T decrease: 0.2 oC/1000 yrs
Orbital cooling
Natural variations of climate
change: orbital scale
-0.2 oC / 1000 yrs = -0.02 oC / 100 years
Overall effect small
(need +0.7 oC / 100 years)
2C. Natural variations of climate
change: “millenial” scale
-- poorly defined
-- possible
variations
on ~1,000 year
scale
Natural variations of climate
change: millenial scale
Perhaps + or -0.2 oC / 1000 yrs =
+ or -0.02 oC / 100 years
overall effect small
(need +0.7 oC / 100 years)
2D. What about
solar output variations?
T measurements
Sunspots
Natural variations of climate
change: solar output
2001 Text: maybe 0.1-0.2 oC / 100 yrs solar warming
2008 Text: estimate 0.07 oC/ 125 yrs = 0.09 oC/100 yrs
could be significant,
but can’t explain all
of the recent warming
(need +0.7 oC / 100 years)
Infer: +0.7 oC/100 yr
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 oC / 100 yrs
to
+0.3 oC / 100 yrs
inferred: 0.7 oC / 100 yrs
residual =
(observed - natural)
= anthropogenic
effect