5
What do changes in the vertical
structure of atmospheric temperature —from the surface up to the
stratosphere—tell us about the causes of recent climate change?
The observed
warming in the lower atmosphere and cooling in the upper atmosphere provide us
with key insights into the underlying causes of climate change and reveal that
natural factors alone cannot explain the observed changes.
In the early
1960s, results from mathematical/physical models of the climate system first
showed that human-induced increases in CO2 would be expected to lead to gradual
warming of the lower atmosphere (the troposphere) and cooling of higher levels
of the atmosphere (the stratosphere). In contrast, increases in the Sun’s
output would warm both the troposphere and the full vertical extent of the
stratosphere. At that time, there was insufficient observational data to test
this prediction, but temperature measurements from weather balloons and
satellites have since confirmed these early forecasts. It is now known that the
observed pattern of tropospheric warming and stratospheric cooling over the
past 40 years is broadly consistent with computer model simulations that
include increases in CO2 and decreases in stratospheric ozone, each caused by
human activities. The observed pattern is not consistent with purely natural
changes in the Sun’s energy output, volcanic activity, or natural climate
variations such as El Niño and La Niña.
Despite this
agreement between the global-scale patterns of modelled and observed
atmospheric temperature change, there are still some differences. The most
noticeable differences are in the tropics, where models currently show more
warming in the troposphere than has been observed, and in the Arctic, where the
observed warming of the troposphere is greater than in most models.
6
Climate is always changing. Why is
climate change of concern now?
All major
climate changes, including natural ones, are disruptive. Past climate changes
led to extinction of many species, population migrations, and pronounced
changes in the land surface and ocean circulation. The speed of the current
climate change is faster than most of the past events, making it more difficult
for human societies and the natural world to adapt.
The largest
global-scale climate variations in Earth’s recent geological past are the ice
age cycles, which are cold glacial periods followed by shorter warm periods.
The last few of these natural cycles have recurred roughly every 100,000 years.
They are mainly paced by slow changes in Earth’s orbit, which alter the way the
Sun’s energy is distributed with latitude and by season on Earth. These orbital
changes are very small over the last several hundred years, and alone are not
sufficient to cause the observed magnitude of change in temperature since the
Industrial Revolution, nor to act on the whole Earth. On ice-age timescales,
these gradual orbital variations have led to changes in the extent of ice
sheets and in the abundance of CO2 and other greenhouse gases, which in turn
have amplified the initial temperature change.
Recent
estimates of the increase in global average temperature since the end of the
last ice age are 4 to 5 °C (7 to 9 °F). That change occurred over a period of
about 7,000 years, starting 18,000 years ago. CO2 has risen more than 40% in
just the past 200 years, much of this since the 1970s, contributing to human
alteration of the planet’s energy budget that has so far warmed Earth by about
1 °C (1.8 °F). If the rise in CO2 continues unchecked, warming of the same
magnitude as the increase out of the ice age can be expected by the end of this
century or soon after. This speed of warming is more than ten times that at the
end of an ice age, the fastest known natural sustained change on a global
scale.
7
Is the current level of atmospheric
CO2 concentration unprecedented in Earth’s history?
The present
level of atmospheric CO2 concentration is almost certainly unprecedented in the
past million years, during which time modern humans evolved and societies
developed. The atmospheric CO2 concentration was however higher in Earth’s more
distant past (many millions of years ago), at which time palaeoclimatic and
geological data indicate that temperatures and sea levels were also higher than
they are today.
Measurements
of air in ice cores show that for the past 800,000 years up until the 20th
century, the atmospheric CO2 concentration stayed within the range 170 to 300
parts per million (ppm), making the recent rapid rise to more than 400 ppm over
200 years particularly remarkable [figure 3]. During the glacial cycles of the
past 800,000 years both CO2 and methane have acted as important amplifiers of
the climate changes triggered by variations in Earth’s orbit around the Sun. As
Earth warmed from the last ice age, temperature and CO2 started to rise at
approximately the same time and continued to rise in tandem from about 18,000
to 11,000 years ago. Changes in ocean temperature, circulation, chemistry, and
biology caused CO2 to be released to the atmosphere, which combined with other
feedbacks to push Earth into an even warmer state.
For earlier
geological times, CO2 concentrations and temperatures have been inferred from
less direct methods. Those suggest that the concentration of CO2 last
approached 400 ppm about 3 to 5 million years ago, a period when global average
surface temperature is estimated to have been about 2 to 3.5°C higher than in
the pre-industrial period. At 50 million years ago, CO2 may have reached 1000
ppm, and global average temperature was probably about 10°C warmer than today.
Under those conditions, Earth had little ice, and sea level was at least 60
metres higher than current levels.
8
Is there a point at which adding more
CO2 will not cause further warming?
No. Adding
more CO2 to the atmosphere will cause surface temperatures to continue to
increase. As the atmospheric concentrations of CO2 increase, the addition of
extra CO2 becomes progressively less effective at trapping Earth’s energy, but
surface temperature will still rise.
Our understanding of the physics by
which CO2 affects Earth’s energy balance is confirmed by laboratory
measurements, as well as by detailed satellite and surface observations of the
emission and absorption of infrared energy by the atmosphere. Greenhouse gases
absorb some of the infrared energy that Earth emits in so-called bands of
stronger absorption that occur at certain wavelengths. Different gases absorb
energy at different wavelengths. CO2 has its strongest heat-trapping band
centred at a wavelength of 15 micrometres (millionths of a metre), with
absorption that spreads out a few micrometres on either side. There are also
many weaker absorption bands. As CO2 concentrations increase, the absorption at
the centre of the strong band is already so intense that it plays little role
in causing additional warming. However, more energy is absorbed in the weaker
bands and away from the centre of the strong band, causing the surface and
lower atmosphere to warm further.
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