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Global Climate Change
The Evidence
and
Human Influence
Principle Evidence
CO2 and Temperature
Temperature Patterns
Climate Change
 Climate is the average weather at a given point and
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time of year, over a long period (typically 30 years).
We expect the weather to change a lot from day to
day, but we expect the climate to remain relatively
constant.
If the climate doesn’t remain constant, we call it
climate change.
The key question is what is a significant change?
This depends upon the underlying level of climate
variability
It is crucial to understand difference between climate
change and climate variability…
Temperature Patterns
Variations of the Earth’s surface temperature
for the past 140 years
Temperature Patterns
Warmest 12 years:
1998,2005,2003,2002,2004,2006,
2001,1997,1995,1999,1990,2000
Period
Rate
50 0.1280.026
100 0.0740.018
Years /decade
Is It Real?
 Earth is getting
warmer by virtually
every measure we
know, and the
temperature has
been well above
normal for more
than 25 years.
Is It Real?
 Although increases of
1.0-1.6oF (0.6-0.9oC)
over the last century or
so may not sound very
threatening, remember
that’s a global
average.
 Area of seasonally
frozen ground in NH
has decreased by 7%
from 1901 to 2002
Spring snow cover
shows 5% stepwise
drop during 1980s
Arctic sea ice
area decreased by
2.7% per decade
(Summer:
-7.4%/decade)
Rise in global ocean heat content
1955-2005
 Earth is getting warmer by
virtually every measure we
know, and the temperature
has been well above
normal for more than 25
years.
 Although increases of 1.01.6oF (0.6-0.9oC) over the
last century or so may not
sound very threatening,
remember that’s a global
average.
 The warming is stronger
over land than over oceans
and in the higher latitudes
than in the tropics.
Sea-level from satellites: 4 cm rise in last 10 years
Could the warming be natural?
 Certainly, past temperatures have been
higher (and lower) than today, and
 CO2 concentrations have also varied.
 Large global swings were probably caused by
such things as changes in Earth’s orbit, which
changed the distribution of sunlight over the
planet.
 When this caused warming, more CO2 and
other greenhouse gases were released,
producing additional warming.
Could the warming be natural?
But today, the CO2 released by human activities is far above amounts
in the previous 800,000 years. This CO2 is triggering the increase in
temperatures we’ve seen.
Mauna Loa CO2 observations
 The data on atmospheric CO2 collected by
Keeling at the Mauna Loa observatory is said
to be the most widely seen data set in the
world.
 It was started in 1955
 CO2 was at 313 ppm
 It was at 389 ppm in 2010 an increase of
19.5% in 55 years
 In February of 2015 it exceeded 400ppm
CO2 Levels
CO2 Levels
Summary (Observations)
 Global surface temperatures have risen by about
0.6°C since 1900
 It is likely that this warming is larger than for any
century since 200AD, and that the 1990s were the
warmest decade in the last millennium.
 The warming differs in different parts of the world, but
over the last 25 years, almost everywhere has
warmed, and very few places have cooled.
 Other changes have occurred
 Sea level has risen by about 20 cm,
 Ocean heat content has increased,
 Almost all mountain glaciers have retreated
Summary (Observations)
 Coincident with this global warming, levels of CO2
(and other ‘greenhouse’ gases) have dramatically
increased, to levels higher than those experienced for
maybe millions of years.
 Question: are temperatures and atmospheric
composition linked?
Global Warming Potentials
• Refers to the relative
ability of one molecule
of a given greenhouse
gas to contribute to
warming.
• Values are expressed in
relation to CO2, which is
assigned a global
warming potential of 1.