VII
UNIT
Global Warming: The Evidence
Learning Objectives
 Analyze anthropogenic global warming from a scientific basis and in the context of
past changes in climate
 Know that the global climate fluctuates on small timescales of years to decades, and
that scientists are still not sure about the causes of these short-term fluctuations
 Know that climate change can occur very rapidly, as evidenced by past rapid
changes in climate such as the Little Ice Age, and the Medieval Warm Period
 Understand the implications these past climate fluctuations have on our analysis of
changes that could occur as a result of anthropogenic global warming
 How do we know that the Medieval Warm Period was a period of unusually warm
summers and the Little Ice Age that followed was an interval of unusually cold
summers?
 Understand how sea-ice can affect interactions between the ocean and atmosphere
Review Questions
1.) 20th Century Warming
How far back in time do direct measurements of Earth’s surface temperature extend?
Since 1861 scientists have made accurate atmospheric temperature measurements using
thermometers.
Why is it difficult to determine accurately the long-term temperature trend?
• Problems exist with historical temperature data. For instance:
Location of stations: Stations located in cities are subject to a “heat island effect” that
causes local temperatures to increase with time.
• Coverage in time and space: Some parts of the world have more coverage than
the others. There is very little coverage, for example, in the southern oceans.
2. What is the Holocene?
The Holocene is the geologic interval from the last glacial retreat to the present. It has
lasted for the past 10,000 years.
3. Why are climatologists usually not very interested in “average” climate fluctuations?
124
“Average” climate fluctuations are not unpredictably large, and therefore the impacts
on society can be mitigated. Large, rapid climate changes are both more
unpredictable and harder to adapt to. Therefore, their impacts on society are more
severe.
4. What are proxy data? Describe several examples of proxy climate data.
Proxy data are data used to determine what a measurable quantity was at a time or
place for which these measurements are unavailable.
5. Describe the major feedback processes between sea ice and climate.
There are three major effects that sea ice has on climate: the melting of sea ice can
slow down deep water formation at high latitudes, impacting global circulation
patters; the formation of sea ice increases the albedo of the Earth, thus decreasing
the surface temperature; and the formation of sea ice decreases the heat flux from
the ocean to the atmosphere, warming the surface.
6. Why do some climate models predict that the thermohaline circulation might shut
down?
In the short term, it is possible that increased warming leads to increased rainfall
in the North Atlantic. Rainfall is a source of fresh water, and as such decreases
the density of North Atlantic surface waters, thus leading to decreased oceanic
downwelling. In the long run, the decreased density of North Atlantic Ocean
waters could be caused by melting of the Greenland Ice Sheet.
Websites:
The Intergovernmental Panel on Climate Change (IPCC), which most consider to be the
consensus view of the scientific community on global warming, has the following
website:
http://www.ipcc.ch/
The Union of Concerned Scientists, a group that seeks to examine environmental
problems from a scientific standpoint, has their hope page at the following address:
http://www.ucsusa.org/
The United States Environmental Protection Agency has the following page on global
warming:
http://yosemite.epa.gov/oar/globalwarming.nsf/content/index.html
125