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This slide show summarizes the scientific consensus on humancaused climate change, as embodied in the United Nationssponsored Intergovernmental Panel on Climate Change 2001 Report
(, the source of the figures shown (annotations added
for this class). The skinny version is, with high confidence:
We are changing atmosphere, especially by burning fossil fuels;
That is changing the climate, primarily by warming;
The changes so far have been tiny compared to the changes that
will come if we follow business as usual;
Changes will cause winners and losers, with losers far
outnumbering winners if warming is large;
Sea-level rise, summer grain-belt drying, and stronger storms are
among the large impacts expected.
Unit 12 - Global Warming
Additional Points on Global Warming:
In the past, environmental problems invariably generated much
noise and denial and claims of huge costs to clean up, until people
invented better ways, after which we solved the problems, people
made money, and the cleanups became part of the economy;
Plausible estimates say completely cleaning up CO2 today would
be very expensive, but could be done after a few decades of serious
research and development for something like 1% of the economy;
That is in line with costs of other cleanups (sewers, janitors,
garbage trucks, catalytic converters…), and, it is a lot of money;
Slowing down emissions a bit now might buy time to avoid
problems until the new technologies are in place;
We will switch energy sources (fossil fuels are finite); will we
switch before or after changing the world, and, who will get rich?
Unit 12 - Global Warming
Rise in greenhouse gases carbon dioxide, methane, and nitrous oxide over last millennium, and rise and
fall in acid-rain sulfate over last 600 years, from ice-core data (sulfate and dots on gas graphs) and
instrumental records (lines for gases). Changes are clearly human-caused. UN IPCC
Unit 12 - Global Warming
Instrumental record (from
thermometers) of global
average temperature from
1860-2000. Corrections
have been made for the
heating effect of cities, so
this shows real warming.
From UN Intergovernmental
Panel on Climate Change
Instrumental record (red) and
proxy-data record (tree-rings,
etc., blue with uncertainty in
gray) of global average
temperature over last 1000
years. Details of proxy
interpretation are still
debated, but, other approaches
give similar results.
Unit 12 - Global Warming
Natural forcing explains temperature changes of a century ago but not recently;
human forcing explains recent changes--Nature mattered, but now we control.
Effects on temperature
of changing sun &
Each panel shows
measured globalaverage
1860-2000 (red
line) and a model
with uncertainty
(gray band).
Effects on temperature
of changing greenhouse
gases & acid rain
Effects on temperature
of changing greenhouse
gases, acid rain, sun &
Unit 12 - Global Warming
Natural causes (more sun,
fewer volcanoes) started
warming (upper left) but
switched to cooling; human
causes (greenhouse gases,
plus acid rain blocking sun)
cooled in 1950s, now
warming strongly (upper
right and bottom).
How much more fossil fuel is
there? We’re sure of reserves; we
think we have resources, and
unconventional reserves and
resources include oil shale, tar
sands and such. Most of the
fossil fuels have not been burned
yet, but we’re burning much
faster now than in the past.
Still there to burn Burned already
Unit 12 - Global Warming
Scenarios show
different possible
futures, depending
on how fast
economy grows and
how hard humans
try to clean up CO2.
to come.
so far.
Past change in CO2 is small compared to change coming, CO2 is still likely to be going
up in 2100, and the world doesn’t end then (you may live past 2100…).
Unit 12 - Global Warming
Uncertainty arises both from
not knowing what humans
will do (different scenarios of
human behavior are used,
shown by different colors),
and from not knowing exactly
how nature will respond to
humans (estimated by running
each scenario through a range
of computer models of the
climate to produce the colorcoded bars on the far right
showing possible changes for
that scenario).
Change to
so far.
Unit 12 - Global Warming
Past temperature
change is small
compared to expected
change for all
scenarios and climate
models tested. Change
so far is small enough
that only those really
paying attention have
noticed; future change
is expected to be much
bigger. All projections
show temperatures
rising beyond the year
2100, which is not the
end of time.
Projected warming
in the greenhouse
future, compared to
changes over the
last millennium.
Again, all the
projection curves
are rising as they
cross the year
2100, and the
world does not end
in 2100.
Unit 12 - Global Warming
Scenario A2 is
one in which
the economy
does pretty
well, and we
don’t try very
hard to clean
up, giving
toward the
upper end of
the scenarios
tested by the
Warming of mean-annual temperature by the year 2100, in degrees Celsius. (To get
Fahrenheit, multiply by 1.8; the largest warmings in the Arctic are almost 10oC or 18oF.)
Unit 12 - Global Warming
Experts were asked how much
warming would worry them about
various things. White means few
were worried, and red means
most were worried. Even a little
warming endangers rare species
(category I)--they need to move
as climate changes, but humans in
the way may block migration.
Much warming is required before
most experts are worried about
ice-sheet or thermohalinecirculation collapse (category V),
but beyond 2100, such warming
becomes increasingly likely
unless actions are taken. How the
economy grows, and how we try
to clean up, determine how hot it
gets, as shown on the right for the
year 2100--burn-it-all makes
enough warming to be worried
about most things by then; slowdown-some leaves a little safety
zone. UN IPCC
Unit 12 - Global Warming
(left) Projected sea-level rise for a range
of human-behavior scenarios and from a
range of models, to the year 2100. Rise
over the last few years seems to have
been faster than any of these
projections. Projected change is from
warming-caused ocean-water expansion
and melting of mountain glaciers.
(right) Number of people likely to be flooded by coastal
storm surges (blue bars) after a 40-cm (16-inch) sea-level
rise (the yellow dotted line on the left) (includes population
growth to the year 2080), versus without sea-level rise
(green), assuming either modern sea-walls, etc., or efforts to
build more sea-walls, etc. Loss of the Greenland ice sheet
would raise sea level over 7 m or 23 feet, and Antarctica
almost ten times more than that. UN IPCC
Unit 12 - Global Warming