Chapter 7: Why is the Earth warming?
Recall:
Climate sensitivity: [2-4.5°C] with a best estimate of 3°C for doubled CO2
If CO2 is doubled from 280-560 ppmv and then the climate runs to equilibrium
Doubled CO2 has a RF of 4 W/m2.
Hence can also express the sensitivity as 3°C /4 W/m2 = 0.75°C/(W/m2)
Q: Why is the climate warming?
There is no “normal” climate; if the climate is cold, it will not “naturally” warm up
There is a physical explanation for every bump and wiggle in this record
List the suspects, evaluate each one in turn …
7.1 Tectonic processes
Arrangement of the continents impacts climate via ice sheets
Varies the planet’s albedo and the distribution of absorption of solar energy
MAP
Distribution matters:
High latitude continents have ice on them, which cools the climate
Ocean circulation can change, which changes heat transport
Example of heat transport:
Opening of Drake passage between South America and Antartica;
Reduced currents from tropics to South Pole
Another example …
Collision of India with Asia
Drives up Tibetan plateau
Creates Asian Monsoon (draw side-view diagram of plateau and how rising air rains)
Rain + CO2 -> H2CO3 (rain is a weak acid)
Weathering of silicate rocks
by rainfall: CaSiO3+CO2+H2O->CaCO3+SiO2
CaSiCO3 is a silicate rock, CaCO3 is limestone, chalk, SiO2 is sand, glass, quartz
CO2 dissolves in rain, making it slightly acidic; rain falls on exposed rock, dissolving it.
The CaCO3 and SiO2 run into the ocean. Draw down atmospheric CO2 causing cooling
Cooling since Eocene: caused (partially) by decrease in CO2 from tectonic activities
“Silicate weathering thermostat”
higher temps -> more rainfall -> removes CO2 -> cooling
clicker: forcing or positive or negative feedback?
But: much too slow to be important for the last few decades
7.2 Solar constant
A. Sun’s output changes; overall increase of 30% since infant sun
Today's sun: 11 – year cycle: variation of about 0.1% every 11 years
Change in solar energy output (monthly)
Effect not seen in global, surface temperatures, ice melt etc.
Effect too rapid: global response time too slow
Sun's output over longer periods? Less well known.
Use proxies (like ice cores) …possible that sun has brightened in last few hundred years
Estimated positive radiative forcing: +0.12 W/m2
But: stratospheric cooling in recent decades; not compatible any increase in sun’s output
So we can rule this out for the recent warming
http://www.atmosphere.mpg.de/enid/20c.html
B. Changes in earth's orbit
Any change in earth-sun distance will change solar constant
Variation in eccentricity of earth's orbit over 100,000 years
Over 100,000 years, eccentricity of orbit changes from min to max
“Milankovitch” cycles
Co-relates very well with ice-age cycles: probable cause
But: much too slow to account for rapid recent changes
Note: orbital variations not enough to lead to large temperature changes in the past
Amplified by changes in greenhouse gases, like CO2 (RHS)
Change in T causes changes in CO2 : feedback loop
This does not mean that CO2 is not a forcing today
C. Change in earth's perihelion
Timing of earth's closest approach to sun changes over time
Currently January – changes 12 months over 20,000 year-cycle
Changes distribution of sunlight – latitudes and seasons
Does not change solar constant globally
Very long cycle
D. Change in earth's angle of tilt
Changes about 2° over 40,000 years
Changes distribution of sunlight – latitudes and seasons
Does not change solar constant globally
Very long cycle
Earth's angle of tilt varies over cycle of 41,000 years
Change in tilt not compatible with stratospheric cooling
7.3 Internal variability
Changes in the climate not associated with external radiative forcing
- known as internal variability or “unforced variability”
Example
El Nino or ENSO:
Duration: one year
Frequency: every few years
Warming: a few tenths of a degree
La Nina: opposite effect
Cannot introduce a long-term trend; cycle too short
Could today’s warming be due to some unknown but long-lived internal oscillation?
Hard to rule out, but no evidence to support such variation
Not compatible with stratospheric cooling
Note: some cooling can be attributed to ozone loss. However, models show enhanced
greenhouse effect should cause overall cooling of stratosphere at high altitude
http://www.atmosphere.mpg.de/enid/20c.html
7.4 Greenhouse gases
Changes in GHGs have been implicated in climate change in the past
Evidence is strong:
1. Theoretical expectation that adding CO2 should warm the climate
2. Paleo evidence; strong correlation between warm temps and high GHG conc.
3. Observation that CO2is indeed increasing (about 40% increase so far)
4. Observation of warming (chapter 2)
Ice ages only at low CO2 concentrations
PETM: good analogue for today’s warming
Several thousand GtC were emitted over a few thousand years
Close-up of PETM
Massive release of CO2 caused temp rise (5-9 oC)
Carbon released caused oceans to become acidic
Calcium carbonate dissolved in ocean sediments
Temperatures remained high for 100,000 years (carbon cycle)
PETM: about the same amount of GHGs if all fossil fuels burnt
but over hundreds of thousands of years (not hundreds)
Correlation between CO2 and temp also evident over last hundred thousand years:
CO2 feedback: initial T perturbation is caused by orbit, amplified by CO2 feedback
Greenhouse gases today:
CO2 rising because of human activities, particularly fossil fuels
Now a “forcing” rather than a feedback
Expect increases in n to warm the planet … if you invoke another theory, then why is
increase in n not causing warming? So you need two theories: one a theory of what is
causing the warming, and another to explain why CO2 is not warming
Simulations; no models that successfully model the past can model the recent
warming without an enhanced greenhouse effect
Simulation of recent warming without enhanced GHG effect
Simulation of recent warming with enhanced GHG effect
IPCC conclusions (2007)
Most of the warming since 1950 very likely due to observed increase in GHG
concentration (90% probability)
Summary
Putting it all together
1. Global warming:
There is unequivocal evidence that the earth’s climate has warmed in the last
100 years. The warming is measured as a rise in the surface temperature of the
earth’s land and oceans, (0.75°C/century, recently 0.13°C/decade ) and in the
deep ocean. The warming is also manifest as a rise in sea level (15
cm/century, recently 0.18 mm/yr) and a loss of sea and land ice.
2. Past climate changes
Studies of past global climate show large variations throughout the history of the
planet. Interglacial periods now well understood in terms of slight changes in the
earth’s orbit, amplified by a feedback effect involving an increase in greenhouse
gases.
Cooling and warming occurred over tens of thousands of years.
Ice ages occurred only at low CO2 concentrations.
3. Causes of recent warming
Natural explanations such as changes in tectonic motion, orbital motion or solar output
cannot account for warming over short time scale. In addition, cooling of stratosphere
difficult to explain
Enhanced greenhouse effect
Simple climate models show importance of greenhouse effect
Expect increased greenhouse conc. to cause warming (forcing)
Evidence of 40% increase in GHG concentration since 1850
Evidence of effect of increased GHG on past climate (feedback)
Expect increased GHG conc. to cause warming in troposphere – observed (above)
Expect increased GHG conc. to cause cooling in stratosphere – observed
Cause of increased GHG conc: burning fossil fuels
Evidence: co-relation with emissions + radiocarbon dating
Conclude: Most of the warming since 1950 very likely due to observed increase in
GHG concentration due to burning of fossil fuels