One of the key questions
about climate change is the strength
of the greenhouse effect. In scientific terms this is described
as "climate sensitivity". It's defined as the amount
Earth's average temperature will ultimately rise in response to a
doubling of atmospheric carbon dioxide levels.
Climate sensitivity has
been hard to pin down accurately. Climate models give a range of
1.5-4.5℃ per doubling of CO₂, whereas historical weather
observations suggest a smaller range of 1.5-3.0℃ per doubling of
According to their
statistical analysis, historical weather observations reveal only a
portion of the planet's full response to rising CO₂ levels. The
only become manifest on a time scale of centuries, due to effects
that researchers call "slow climate feedbacks".
To understand this, it is
important to know precisely what we mean when we talk about climate
sensitivity. So-called "equilibrium climate sensitivity",
or slow climate feedbacks, refers to the ultimate consequence of
climate response – in other words, the final effects and
environmental consequences that a given greenhouse
These can include
long-term climate feedback processes such as ice sheet disintegration
with consequent changes in Earth's surface reflection (albedo),
changes to vegetation patterns, and the release ofgreenhouse
as methane from soils, tundra or ocean sediments. These processes can
take place on time scales of centuries or more. As such they can only
be predicted using climate
models based on prehistoric data and paleoclimate evidence.
Estimate of climate
forcing for 1750-2000. Author provided
On the other hand,
when greenhouse gas
forcing rises at a rate as high as 2–3 parts per million (ppm) of
CO₂ per year, as is the case during the past
decade or so, the rate of slow feedback processes may be
atmosphere and marine changes made since the Industrial Revolution
(when humans first began the mass release of greenhouse gases)
capture mainly the direct warming effects of CO₂, as well as
short-term feedbacks such as changes to water vapour and clouds.
A study led
by climatologist James Hansen concluded that climate
sensitivity is about 3℃ for a doubling of CO₂ when considering
only short-term feedbacks. However, it's potentially as high as 6℃
when considering a final equilibrium involving much of the West and
East Antarctic ice melting, if and when global greenhouse levels
transcend the 500-700ppm CO₂ range.
This illustrates the
problem with using historical weather observations to estimate
climate sensitivity – it assumes the response will be linear. In
fact, there are factors in the future that can push the curve upwards
and increase climate variability, including transient reversals that
might interrupt long-term warming. Put simply, temperatures have not
yet caught up with the rising greenhouse gas levels.
So far we have
1.1℃ of average global warming since the Industrial
Revolution. Over this time atmospheric CO₂ levels have risen from
280ppm to 410ppm –
and the equivalent
of more than 450ppm after factoring in the effects of all
the other greenhouse gases besides CO₂.
The growth in the cold
water region south of Greenland, heralding a possible collapse of the
Atlantic Mid-Ocean Circulation. Author provided
Climate change is
unlikely to proceed in a linear way. Instead, there is a range of
potential thresholds, tipping points, and points of no return that
can be crossed during either warming or transient short-lived cooling
pauses followed by further warming.
Such events could include
the collapse of the Atlantic Mid-Ocean Circulation (AMOC), with
consequent widespread freezing associated with influx of extensive
ice melt from the Greenland and other polar ice sheets. The influx of
cold ice-melt water would abort the warm salt-rich AMOC, leading to
regional cooling such as is recorded following each
temperature peak during previous interglacial periods.
A collapse of the AMOC,
which climate "sceptics" would no doubt welcome as
"evidence of global cooling", would represent a highly
disruptive transient event that would damage agriculture,
particularly in the Northern Hemisphere. Because of the cumulative
build-up of greenhouse gases in the atmosphere such a cool pause is
bound to be followed by resumed heating, consistent with IPCC
Humanity's release of
greenhouse gases is unprecedented in speed and scale. But if we look
far enough back in time we can get some clues as to what to expect.
million years ago, Earth experienced warming by 5-8℃ lasting
several millennia, after a sudden release of methane-triggered
feedbacks that caused the CO₂
level rise to around 1,800ppm.