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Past Climates
Estimated Phanerozoic Temperatures
(14° is Today; 16° is Critical and 18° is Catastrophic for
Generalized Climates for the Past 3 Billion Years
Climate Change During Past 180 Million Years
Cenozoic Era
End of Cretaceous (65 My BP)
Present Day
The Paleocene-Eocene Thermal Maximum (PETM)
The Pliocene/Pleistocene “Ice House”
Summary: Cenozoic Era
1. Dominant Forcing: Natural Differences in CO2
- Rate ~100 ppm/Million years (0.0001 ppm/year)
- Human-made rate today: ~2 ppm/year (20,000
times faster than the natural rate)
Humans Overwhelm Slow Geologic Changes
2. Climate Sensitivity High
- Antarctic ice forms if CO2 < ~450 ppm
- Ice sheet formation reversible
Humans Could Produce “A Different Planet”
Change in Sea Level During the Last Glacial
and Interglacial Periods
Sea Level in North America if all Ice
on Earth Melted
Extent of the Ice Sheet that Covered North America
during the Last Ice Age
Temperature Variations During
the Past 140,000 Years
Abrupt Climate Change:
Our Worst Nightmare
Variations in Temperature During part
of the Last Ice Age
The Younger Dryas (YD) and Other
Abrupt Climate Changes
Several Abrupt Climate Changes
Nine current tipping elements vulnerable to possible abrupt change. The time
frames and threshold temperature increases may be modified with more data.
Indian Summer Monsoon
The regional atmospheric brown cloud is one of
the many climate change-related factors that could
disrupt the monsoon.
One Year
Sahara and West African
Small changes to the monsoon have triggered
abrupt wetting and drying on the Sahara in the
10 Years
3-5° C
Arctic Summer Sea-ice
As sea ice melts it exposes darker ocean, which
absorbs more heat than ice does, causing further
10 Years
0.2-4° C
Amazon Rainforest
Losing critical mass of the rainforest is likely to
reduce internal hydrological cycling, triggering
further dieback.
50 Years
3-4° C
Boreal Forests
Longer growing seasons and dry periods increase
vulnerability to fires and pests.
50 years
3-5° C
Atlantic Ocean
Thermohaline Circulation
Regional Ice melt will freshen North Atlantic water,
shutting down the ocean circulation.
100 years
3-5° C
El Nino-Southern
Oscillation (ENSO)
Climate models suggest ENSO will enter a nearpermanent switch-on raising temperatures.
100 years
3-6° C
Greenland Ice Sheet
Break-up of ice sheet raising sea level drastically.
300 years
1-2° C
West Antarctic Ice Sheet
Ice sheet is frozen to submarine mountains, so
high potential for sudden release and collapse as
oceans warm.
300 years
3-5° C