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Climate Change: The Latest
Scientific Assessment
Emerging Growth Issues Workshop
June 27, 2001
Anthony D. Socci, Ph.D.
U.S. Environmental Protection
Key Points
• Is there a greenhouse effect? Yes, the existence of a natural
greenhouse effect is well established. Heat-trapping greenhouse gases
such as CO2, CH4, water vapor and N2O prevent a significant portion
of incoming solar radiation from escaping back into space resulting in
warming at the Earth’s surface. In the absence of these gases, the
Earth’s average temperature would be 60 º F (33.4 ° C) cooler.
• Have greenhouse gases increased as a consequence of human
activities? Yes, greenhouse gases such as CO2, CH4, and N2O have
increased considerably since 1750, the start of the industrial revolution.
In the case of carbon dioxide and methane, present levels of these
gases have not been exceeded in the last 420,000 years.
• Are we experiencing a global warming? Yes, the global average
temperature of the Earth’s surface has warmed by about 1.0 º F (0.6 °
C) since 1860, mostly during the latter half of the 20th Century. The
warming is real and particularly strong within the last 20 years.
• Is this warming due to the increase in greenhouse gas concentrations?
Most of the observed warming over the last 50 years is likely to be due
to the increase in greenhouse gas concentrations.
Key Points Continued
• Are human activities contributing to a global warming? The warming
over the past 100 years is very unlikely to be due to internal climate
variability alone, and it is unlikely to be entirely natural in origin.
Current thinking suggests that human activities are responsible for at
least part of the observed warming, especially over the latter half of the
20th Century.
• Global warming could well have serious adverse societal and
ecological impacts by the end of this century, especially at the upper
end of the IPCC temperature projections.
• Even in the more conservative scenarios, models project temperatures
and sea-levels that continue to increase well beyond the end of this
century, suggesting that assessments that only examine the next 100
years may well underestimate the magnitude of the eventual impacts.
• Risk increases with increases in both the rate and the magnitude of
climate change.
Sources: IPCC WG I Summary for Policy-Makers, Third Assessment Report,
2001; Climate Change Science, National Academy of Sciences, 2001.
Human Influence
on the Atmosphere during the Industrial Era
CO2 up 31%
CH4 up 151%
N2O up 17%
Different symbols denote ice core data for
several sites in Antarctica and Greenland
Source: IPCC WG I (Science) Summary
for Policy-Makers, Third Assessment
Report, 2001.
Temperature and CO2 concentration over the last 420,000 years. The present atmospheric CO2
concentration is 367 ppmv (parts per million by volume), the highest level seen in the last 420,000
Source: Petit, Jouzel, et al, Nature, 1999.
Observed globally-averaged temperatures, 1880-2000. The mean annual temperature
has risen by about 0.6 ˚C (1.2 ˚F) since 1880. The eight warmest years in this modern
instrumental record occurred in the decade of the 1990s and are listed here in
descending order - 1998, 1997, 1995, 1990, 1999, 2000, 1991 and 1994.
Source: NOAA National Climate Data Center.
Observed and Reconstructed Surface Temperature
Anomalies for the Northern Hemisphere Over the Past
1,000 Years
The rate and duration of warming in the 20th century has been much greater than
in any of the previous nine centuries. It is likely that the 1990s have been the
warmest decade, and 1998 the warmest year of the millenium. The gray region
represents the 95% confidence range.
Source: IPCC WG I Summary for Policy-Makers, Third Assessment Report, 2001.
Simulated Annual Global Mean Surface
Source: IPCC WG I (Science)
Summary for Policy-Makers,
Third Assessment Report,
The Global Climate of the 21st Century
Range of Projected Outcomes from the IPCC Special Report on Emission Scenarios (SRES)
40 Total Scenarios
Source: IPCC WG I (Science) Summary for Policy-Makers, Third Assessment Report, 2001.
Variations of the Earth’s Surface
Temperature: 1000 to 2100
• 1000 to 1861, N.
Hemisphere, proxy data;
• 1861 to 2000 Global,
• Gray shaded areas
represent 95% confidence
range (through 2000)
• 2000 to 2100, SRES
Source: IPCC WG I (Science)
Summary for Policy-Makers,
Third Assessment Report, 2001
Plausible Impacts
• Higher maximum temperatures and more hot days
and heat waves over land areas (VL).
• Higher minimum temperatures and fewer cold
days, frost days and cold waves (VL).
• More intense precipitation events over many areas
• Increased summer drying over most mid-latitude
continental interiors and associated risk of drought
• Intensified droughts and floods associated with El
Nino events in many different regions (L).
Note: Judgmental estimates - VL = 90 - 99% chance; L = 66 - 90% chance
that result is true.
Plausible Impacts for the U.S.
• Reduced lake levels and outflows for the Great
Lakes/St.Lawrence (MC).
• Enhanced coastal erosion, flooding, loss of wetlands and
increased storm surges especially off FL and much of the
Atlantic coast (HC).
• Vector-borne diseases may expand ranges in North
America (MC).
• Exacerbated air quality and heat stress morbidity and
mortality may occur (MC).
• Weather-related insured losses and public sector disaster
relief payments in N.America have been increasing potential for surprises (HC).
Note: Judgmental estimates - HC = 67 - 95%; MC = 33 - 67%.
Source: IPCC SPM, Third Assessment Report, Climate Change 2001: Impacts,
Adaptation and Vulnerability, 2001.
Number of Billion Dollar Weather
Source: NOAA/National Climate Data Center, 2000
Unresolved Issues
• How much warming will occur?
• How fast will the planet warm?
• How much of the warming is natural and
how much is human-induced?
• What are the regional details of how climate
change will manifest itself in the future?
• Other sources of uncertainty: clouds and the
role of water vapor in the upper atmosphere,
• Models do some things well.
Administration’s Position on Global
Climate Change
• The President takes global climate change very
seriously. He is committed to addressing the issue in
a manner that protects our environment, consumers
and economy.
• The U.S. opposes the Kyoto Protocol because it
exempts many countries from compliance and would
cause serious harm to the U.S. economy.
• The Administration is undertaking a Cabinet-level
review of U.S. climate change policy. This review will
consider what policies this Administration should
pursue domestically and internationally.
• We are optimistic that, working constructively with our
friends and allies through international processes, we
can develop technologies, market-based incentives,
and other innovative approaches to global climate