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22 October 2015
From the section Science & Environment
In December, officials from across the world will gather in Paris, France, to
try to hammer out a deal to tackle global warming. Here's what we know and
don't know about the Earth's changing climate.
What is climate change?
The planet's climate has constantly been changing over geological time. The
global average temperature today is about 15C, though geological evidence
suggests it has been much higher and lower in the past.
However, the current period of warming is occurring more rapidly than many past
events. Scientists are concerned that the natural fluctuation, or variability,
is being overtaken by a rapid human-induced warming that has serious
implications for the stability of the planet's climate.
What is the "greenhouse effect"?
The greenhouse effect refers to the way the Earth's atmosphere traps some of
the energy from the Sun. Solar energy radiating back out to space from the
Earth's surface is absorbed by atmospheric greenhouse gases and re-emitted in
all directions.
The energy that radiates back down to the planet heats both the lower
atmosphere and the surface. Without this effect, the Earth would be about 30C
colder, making our planet hostile to life.
Scientists believe we are adding to the natural greenhouse effect with gases
released from industry and agriculture (known as emissions), trapping more
energy and increasing the temperature. This is commonly referred to as global
warming or climate change.
The most important of these greenhouse gases in terms of its contribution to
warming is water vapour, but concentrations show little change and it persists
in the atmosphere for only a few days.
On the other hand, carbon dioxide (CO2) persists for much longer (it would take
hundreds of years for it to return to pre-industrial levels). In addition,
there is only so much CO2 that can be soaked up by natural reservoirs such as
the oceans.
Most man-made emissions of CO2 are through the burning of fossil fuels, as well
as through cutting down carbon-absorbing forests. Other greenhouse gases such
as methane and nitrous oxide are also released through human activities, but
their overall abundance is small compared with carbon dioxide.
Since the industrial revolution began in 1750, CO2 levels have risen by more
than 30% and methane levels have risen more than 140%. The concentration of CO2
in the atmosphere is now higher than at any time in at least 800,000 years.
What is the evidence for warming?
Temperature records going back to the late 19th Century show that the average
temperature of the Earth's surface has increased by about 0.8C (1.4F) in the
last 100 years. About 0.6C (1.0F) of this warming occurred in the last three
decades.
Satellite data shows an average increase in global sea levels of some 3mm per
year in recent decades. A large proportion of the change in sea level is
accounted for by the thermal expansion of seawater. As seawater warms up, the
molecules become less densely packed, causing an increase in the volume of the
ocean.
But the melting of mountain glaciers and the retreat of polar ice sheets are
also important contributors. Most glaciers in temperate regions of the world
and along the Antarctic Peninsula are in retreat. Since 1979, satellite records
show a dramatic decline in Arctic sea-ice extent, at an annual rate of 4% per
decade. In 2012, the ice extent reached a record minimum that was 50% lower
than the 1979-2000 average.
The Greenland Ice Sheet has experienced record melting in recent years; if the
entire 2.8 million cu km sheet were to melt, it would raise sea levels by 6m.
Satellite data shows the West Antarctic Ice Sheet is also losing mass, and a
recent study indicated that East Antarctica, which had displayed no clear
warming or cooling trend, may also have started to lose mass in the last few
years. But scientists are not expecting dramatic changes. In some places, mass
may actually increase as warming temperatures drive the production of more
snows.
The effects of a changing climate can also be seen in vegetation and land
animals. These include earlier flowering and fruiting times for plants and
changes in the territories (or ranges) occupied by animals.
What about the pause?
In the last few years, there has been a lot of talk about a pause in global
warming. Commentators argued that since 1998, there had been no significant
global warming despite ever increasing amounts of carbon dioxide being emitted.
Scientists have tried to explain this in a number of ways.
These include:
variations in the Sun's energy output
a decline in atmospheric water vapour
greater storage of heat by the oceans.
But so far, there is no general consensus on the precise mechanism behind the
pause.
Sceptics highlight the pause as an example of the fallibility of predictions
based on computer climate models. On the other hand, climate scientists point
out that the hiatus occurs in just one component of the climate system - the
global mean surface temperature - and that other indicators, such as melting
ice and changes to plant and animal life, demonstrate that the Earth has
continued to warm.
How much will temperatures rise in future?
In its 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC)
forecast a global temperature increase of between 1.8C and 4C by 2100.
Even if we cut greenhouse gas emissions dramatically now, scientists say the
effects will continue because parts of the climate system, particularly large
bodies of water and ice, can take hundreds of years to respond to changes in
temperature. It also takes greenhouse gases decades to be removed from the
atmosphere.
How will climate change affect us?
The scale of potential impacts is uncertain. The changes could drive freshwater
shortages, bring sweeping changes in food production conditions, and increase
the number of deaths from floods, storms, heat waves and droughts. This is
because climate change is expected to increase the frequency of extreme weather
events - though linking any single event to global warming is complicated.
Scientists forecast more rainfall overall, but say the risk of drought in
inland areas during hot summers will increase. More flooding is expected from
storms and rising sea levels. There are, however, likely to be very strong
regional variations in these patterns.
Poorer countries, which are least equipped to deal with rapid change, could
suffer the most.
Plant and animal extinctions are predicted as habitats change faster than
species can adapt, and the World Health Organization (WHO) has warned that the
health of millions could be threatened by increases in malaria, water-borne
disease and malnutrition.
As an increased amount of CO2 is released into the atmosphere, there is
increased uptake of CO2 by the oceans, and this leads to them becoming more
acidic. This ongoing process of acidification could pose major problems for the
world's coral reefs, as the changes in chemistry prevent corals from forming a
calcified skeleton, which is essential for their survival.
Computer models are used to study the dynamics of the Earth's climate and make
projections about future temperature change. But these climate models differ on
"climate sensitivity" - the amount of warming or cooling that occurs as a
particular factor, such as CO2. goes up or down.
Models also differ in the way that they express "climate feedbacks".
Global warming will cause some changes that look likely to create further
heating, such as the release of large quantities of the greenhouse gas methane
as permafrost (permanently frozen soil found mainly in the Arctic) melts. This
is known as a positive climate feedback.
But negative feedbacks exist that could offset warming. Various "reservoirs" on
Earth absorb CO2 as part of the carbon cycle - the process through which carbon
is exchanged between, for example, the oceans and the land.
The question is: how will these balance out?