Anthropogenic Climate Change:
Past, Present, and Future
Dr. David Karowe
Department of Biological Sciences
Earth’s climate is always changing
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Antarctic ice cores provide a record of Earth’s climate
over the past 800,000 years
- air bubbles trapped in ice reveal past CO2 levels
Over the past 800,000 years, Earth has experienced
repeated ~100,000 yr climate cycles
- greenhouse gasses have co-varied with temperature
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Pleistocene climate cycles are due to
Milankovitch Cycles
Last Glacial Maximum was about 18,000 years ago
- global temperature was only about 5o C cooler than today
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and sea level was about 350 feet lower
Tree ring and thermometer records indicate two
anomalous periods since 500 AD
modern
warming
Little Ice Age
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Video included here
see 18,000 bp – present at
http://homepages.wmich.edu/~karowe/Teaching%202010.html
Animation: North America 19,000 BC to 3,000 AD
“Little Ice Age” from ~1500-1800 AD
- global temperature was ~ 1o C cooler than today
- most of Europe was noticeably cooler
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Little Ice Age was due to decreased solar activity during
the Maunder Minimum
Tree ring and thermometer records indicate two
anomalous periods since 500 AD
modern
warming
Little Ice Age
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Climate change since the Industrial Revolution
Since 1900, Earth has warmed ~ 0.8o C
12 warmest years in history:
2002-2012, 1998*
Warmest year: 2010
- warming is now 20x faster than any time in ice cores
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Temperature difference relative to 1950-1980
video available at http://data.giss.nasa.gov/gistemp/animations/
Precipitation is a very important component of climate
- severe rainstorms have become more common
- 20% increase in Michigan since 1948
- 50% increase in southwest Michigan
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Flooding has increased globally in the last 50 years
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2000-2009
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Much of the world is also already experiencing
more frequent and stronger droughts
1950-1959
Palmer Drought Severity Index (PDSI)
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Arctic summer sea ice has been decreasing rapidly
August 27, 2012
- record low in 2012
Greenland ice sheets is melting at an accelerating rate
2012
- meltwater lubricates underside of
ice sheet
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Major climate predictions are already coming true
in the U.S.
- in 2011, 10 states had their wettest spring on record
As a result, there was extensive flooding
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2012 was the warmest year on record for 19 states
- and for the entire contiguous U.S.
This year, severe drought covered much of the U.S.
December 2012
74% drought
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Can climate change be due to “natural variation”?
- three lines of evidence argue that current climate
change is NOT due to natural factors
1. Since 1900, heat added by greenhouse gasses
is about 20 times heat added by the sun
heat added by
greenhouse gasses
heat added by the sun
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Solar Irradiance
2. While Earth has been warming fastest, energy
from the sun has decreased
3. Models do a good job of replicating past climate
change only if they include anthropogenic factors
anthropogenic
and natural factors
modeled
natural factors only
observed
- natural factors would have caused a slight cooling
since 1900
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In two major ways, current warming is
very different than any warming period
in at least the last 800,000 years
1. It’s at least 20 times faster
2. It’s happening while solar input is decreasing
But, since climate has changed before, it must be
changing now for the same reason, right?
Would the same reasoning apply to recent increases in
forest fires and lung cancer?
Best estimate: at least 95% of current warming is
due to human activities
Greenhouse gas emissions
Deforestation
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What does the future hold?
It depends on the choices we make
Depending on our actions, Earth is likely to warm by
at least 2-4o C by 2100
“Business
as usual”
4o
2o
0.6o
Alternate
energy
sources
Zero emissions
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Will a 4o temperature rise matter?
When Earth was 5o cooler:
Arctic is likely to warm at about twice the global rate
e.g. with 3.5o C average global warming by 2100:
- Arctic is site of strongest positive feedbacks
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Arctic summer sea ice expected to disappear by 2070
For the video include here,
see present – 1,000 years into the future at:
http://homepages.wmich.edu/~karowe/Teaching%202010.html
Animation: North America 19,000 BC to 3,000 AD
Many glaciers are likely to disappear by 2100,
including all intertropical glaciers by 2030
- 750,000,000 people in China and India rely on glaciers
and seasonal snowpacks for their water supply
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Continued loss of ice sheets
- full melting of Greenland ice sheet → 7 m sea level rise
- full melting of Antarctic ice sheet → 57 m sea level rise
Sea level is expected to rise by 1-2 meters by 2100
- 10 million to 300 million people could be flooded each
year with 1 meter rise
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Much of U.S. would be affected by 1 meter rise
Increase in inland flooding due to severe rainfall
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Decrease in summer rainfall throughout the U.S.
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2060-2069
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Much of the world is likely to experience much more
frequent and stronger droughts by the 2060s
Palmer Drought Severity Index (PDSI)
- current drought indices may no longer work properly
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Michigan will experience many more days over 90o F
Most summers are likely to be hotter than any
experienced thus far
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Many areas are also predicted to experience
“disappearing climates” by 2100
Low
Moderate
High
Probability of disappearing climate
Bottom Line:
1. We’re causing climate to change at an
alarming rate
2. If we continue on our current path, the future
is very likely to include unprecedented
hardships for all of Earth’s species, including
humans
3. Many of the worst scenarios can be avoided by
choices we have available today
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