Summer’s On The Way: How’s Earth’s Temperature?
With Summer on the way – Yippy – and an unseasonally hot spell
for March/April so far in the UK, I thought I’d post a blog on Temperature,
taken from Chapter T of THE A-Z OF GLOBAL WARMING.
Temperature is generally measured using the Celsius scale,
except in the USA, where the Fahrenheit scale is used. Zero degrees
Centigrade corresponds to the temperature at which water freezes,
and 100 degrees when it boils. These temperatures are represented
as 32 and 212°F respectively.
The Earth’s average temperature, assisted by its naturally
occurring greenhouse-gas blanket, is about 15°C (59°F). The
average temperature of the human body is about 37°C (98°F), and
if temperatures get too high harmful reactions and even death
may result.
Just like a human being, if Earth’s temperature increases too
much, the planet will start to get sick and serious consequences
will result, some of which are already becoming evident.
How much has the Earth’s temperature increased?
The Earth’s global mean surface temperature according the Fourth
Assessment Report of the IPCC puts the rise at 0.74°C (1.33°F)
over the period 1906 to 2006.
Global temperature is measured by taking the average near-surface
temperatures over air, sea and land.
This rise may not seem like much, but according to NASA,
this means that the Earth is now reaching and passing through
the warmest period in the current interglacial period, which has
lasted for nearly 12,000 years.
How fast is Earth’s temperature rising?
The Earth’s temperature has risen by about 0.2°C (0.36°F) each
decade over the last thirty years. The studies show that warming
is greatest at higher latitudes of the northern hemisphere, and
larger over land compared to the oceans, as the oceans have a
much higher heat capacity compared to the land. Air temperatures
in the Arctic region for example have, on average, actually
increased by about 5°C (9°F) over the last 100 years.
What about historical warming?
We know from Chapter H that the Earth has had many periods
of warming and cooling, and historically these temperature
changes have had little to do with manmade greenhouse gases,
as mankind has been emitting greenhouse gases significantly only
since the Industrial Revolution, in and about the late nineteenth
century.
Two of the most recent temperature changes took place during
the Little Ice Age, in the years 1350–1850, or thereabouts, when
temperatures dipped, and the Medieval Warm Period between
years 1000–1300, or thereabouts, when temperatures got
comparatively warmer again. An explanation for the Little Ice
Age, or Maunder Minimum is the lack of sunspot activity and
solar irradiance that occurred during this time .
What about more recently?
Well, temperatures have been measured accurately with scientific
instruments for about only 150 years or so. Prior to this a range
of proxy data is used, such as tree rings, ice cores, lake and sea
sediments, corals and historical records.
Researchers from NASA, Dr James Hanson and his colleague
Mark Imhoff, analysed records from 7,500 global weather stations
and used satellite observations of night-time weather stations to
identify minimal human influence, such as urban heat island effects.
The team concluded that from 1900 to 1940 it was possible the
Earth warmed partly as a result of increased levels of greenhouse
gases and partly due to natural climate variability.
Between 1940 and 1965 the Earth cooled by about 0.1°C (0.18°F),
which some scientists attribute to the increased use of aerosols
and other airborne pollutants from the burning of fossil fuels.
This was especially so in the northern hemisphere, where cooling
occurred most during this period, which can lead to increased
cloud cover, which in turn blocks and reflects incoming solar
radiation. This is a phenomenon that has been termed ‘global
dimming’. Aerosols, certainly in the northern hemisphere, have
been slowly phased out however, which may have helped reveal
the true extent of greenhouse-gas-induced warming.
The period from 1965 to 2000 showed large and widespread
warming around the world.
Indeed the IPCC concluded in 2001 that there is new and stronger
evidence that most of the warming observed at least over the past fifty
years is attributable to human activities.
Link between global warming and human activities?
There has been much debate between scientists over attribution
of climate change and global warming, and much of this discussion
has focused on a temperature graph produced in 1999 for the IPCC,
by climatologist Michael Mann and his colleagues, which showed
temperatures extending back 1,000 years. The debate became
known as the ‘hockey stick’ debate.
This name came from the graph itself, as it shows temperatures
for about 1,000 years remaining more or less constant, then from
about 1800 a sharp upward trend occurs that resembles the end
of a hockey stick.6 The reconstructions showed the 1990s to be
the warmest decade, with 1998 the warmest year ever.
The graph seems to support the warming influence human
beings have had on climate over the last 150 years or so, as
evidenced by the sudden upward trend in temperatures recorded.
Certain criticism was made of the fact that accurate temperature
records go back only 150 years, and that the data and methods
used to recreate the temperature prior to about 1850 cannot be
reliable as it comes from proxy sources such as tree rings, corals
and ice cores, etc.
It would appear however that much of the debate as to who
is responsible for global warming is now settled. While solar
intensity and even volcanoes and other natural factors can explain
variations in global temperatures in the early nineteenth century,
rising greenhouse gas levels can provide the only plausible
explanation for the warming trend over the past fifty years.7
In response to the controversy over the Mann temperature
graph, in 2006 the US Congress requested the National Research
Council prepare a report. They concluded that there was a high
level of confidence that the global mean surface temperature
during the past few decades is higher now than at any time over
the preceding 400 years. There is less confidence prior to the year
1600 to support temperature reconstructions, as there is less data
available from whatever source. There was even less confidence
about the conclusions reached that the 1990s were the warmest
decade and 1998 the warmest year. The committee did indicate,
however, that none of the reconstructions showed that
temperatures were warmer during medieval times than during
the last few decades.
The main conclusion, however, is that the build-up of
greenhouse gases in the atmosphere will cause several degrees
of warming, and this is based on the laws of physics and chemistry.
The link between greenhouse gases and temperature is well
established, as we know from Chapter G, so when additional
CO2 is added to the atmosphere, by burning fossil fuels, the
temperature is going to increase. This has been confirmed by
reliable scientific instruments over the last 150 years.
How high will temperatures go?
For the last three decades temperatures have been rising by about
0.2°C (0.36°F) per decade. There is evidence however that the
warming may accelerate as positive feedback mechanisms come
into play. Examples would be the release of methane from the
ground as the permafrost starts to melt, thus accelerating the
warming. Studies already indicate that warming is greater over
the northern hemisphere. As the snow and ice melt in the Arctic
regions, darker surfaces are uncovered, which reduces the albedo
effect of the ice/snow-covered areas, which allows more sunlight
to be absorbed, thus increasing warming. Likewise as the
atmosphere warms it is able to hold more water vapour (itself a
greenhouse gas), which allows it to trap more heat. These are two
examples of positive feedback mechanisms.
It is not yet possible however to determine what temperature
will result from a certain level of greenhouse gas.
It is estimated that if greenhouse gas could be stabilised at
today’s level of about 430 ppm CO2 equivalent, the Earth would
be committed to an eventual temperature increase of about 1–
3°C (1.8–5.4°F) above pre-industrial levels.
Projected CO 2 /temperature level scenarios
The amount the Earth’s temperature goes up depends on
greenhouse gas levels in the atmosphere.
Projections of future warming depend on projections of global
emissions. If emissions were to remain at today’s levels, then
greenhouse gas would reach about 550 ppm CO2e by about 2050,
based on the current annual increase of 2.5 ppmv CO2e. This would
commit the world to a temperature rise of about 2–5°C (3.6–9°F).
The IPCC however projects that without intervention
greenhouse gas levels will rise to 550–700 ppm CO2e by 2050,
and 650–1,200 ppm CO2e by 2100! This would cause temperature
rises of between 1.5–4.4°C (2.7–7.9°F) and 1.8–5.5°C (3.2–9.9°F)
respectively, just on the lower forecasts of 550 and 650 ppm CO2e
levels alone!
‘A temperature rise of 2–3°C (3.6–5.4°F) above present
levels would put the Earth at a temperature not
experienced for three million years and far outside the
experience of human civilisation.’
The Earth is already committed to a 1–3°C rise (1.8–5.4°F) on
current greenhouse gas levels. If the Earth warms by a further
1°C (1.8°F), NASA scientists point out that this will be the warmest
Earth has been for the past 1,000,000 years. At 2 or 3°C higher
(3.6–5.5°F), the Earth would become a different world from that
we know. As mentioned above, the last time this occurred was
about 3,000,000 years ago, and sea levels are estimated to have
been twenty-five metres higher (eighty feet) than present!
There seems to be no alternative therefore other than
humankind reduce greenhouse gas emissions, significantly, and
fast, in order to prevent disastrous consequences. The big
problem is that like a huge oil tanker trying to make a U-turn,
even if emissions could be halted now, the effects of current
levels will continue to cause temperatures to rise for a long time
to come.
Any evidence of increasing temperatures currently
affecting Earth?
According to the WWF, evidence comes from the bleaching and
degradation of coral reefs (discussed further in Chapter V), due
to increasing sea temperatures, which could degrade Australia’s
Great Barrier Reef in a single human lifetime. Alpine forests
struggle to spread to higher, cooler locations, and glaciers are
melting all over the world.
The Caribbean saw its warmest ever ocean temperature in 2005.
Scotland in the UK saw its hottest year on record in 2003, which
caused hundreds of adult salmon to die, as the water became too
warm for the fish to extract oxygen from it.
New modelling work by the UK’s Hadley Centre shows that
the summer of 2003 was Europe’s hottest for 500 years.
In the Arctic, sea ice measurements in 2007 recorded the smallest
sea-ice cover ever at the end of the summer melt season.
In 2003, the world’s major cities sweltered under heatwaves.
In France, during the summer of 2003, the heatwave killed about
14,800 people in Paris alone, according to official figures released
in September 2003.
Summer temperatures have been analysed in sixteen of Europe’s
cities, which show that the continents’ capitals have warmed by
up to 2°C (3.6°F) in the last thirty years.
London is the city where average maximum summer
temperatures increased the most, up 2°C (3.6°F) over the last
thirty years, followed by Athens and Lisbon (1.9°C or 3.4°F),
Warsaw (1.3°C or 2.3°F) and Berlin (1.2°C or 2.1°F).16
Between 2000 and 2005, average summer temperatures in
thirteen out of sixteen cities looked at were at least 1°C (1.8°F)
higher than during the period 1970–1975.
Earth’s warmest years
According to climatologists at NASA’s Goddard Institute for Space
Studies the five warmest years since the 1880s have been;
1 2005/2010
2 1998
3 2002
4 2003
5 2006
The year 2005 and 2010 therefore have been the hottest so far, though they
share this accolade with 1998, which was virtually as hot. Year
1998 temperatures were enhanced however by the strongest
tropical El Niño for almost a century, which boosted temperatures
above the level they otherwise would have been. As the El Niño
gets underway in the topical Pacific Ocean, 2007 could be even
hotter, bringing with it increased warmth.
A 2°C (3.6°F) increase limit
The WWF is advocating that temperatures cannot be allowed to
rise by more than 2°C (3.6°F) above pre-industrial levels, otherwise
dangerous climate change may occur. The Earth has already
warmed by 0.74°C (1.33°F), which means another 1.3°C rise (2.34°F)
could be too much.
The 2°C (3.6°F) threshold is based on the best available science
and is accepted by many governments including the prime
ministers and presidents of all twenty-five EU member states.
The only way to prevent temperatures staying below this level
is for CO2 concentrations to stay below about 400 ppmv, the
equivalent to greenhouse gas levels of around 450co2e. If this were
possible, staying below 2°C (3.6°F) is likely, according to climate
models. Levels of CO2 however are already at 395 ppmv, which
means the chance of stabilisation below 400 ppmv is therefore
very unlikely.
What would a 2°C (3.6°F) rise in temperature
mean?
The WWF has looked at three regions to see what a 2°C (3.6°F)
temperature rise would mean for those regions.
The Mediterranean
Everyone enjoys going on holiday to the ‘Med’, with its beautiful
warm climate. However, as temperatures rise in the region, water
shortages could become common as annual rainfall could decrease
by twenty per cent, and more heat-waves cause all-year-round risk
from serious forest fires, as maximum temperatures could rise
by up to 5°C (9°F).
The Arctic
Temperatures would rise by about 3.2°C (5.7°F) here, maybe even
double that if temperatures rose by 2°C (3.6°F) elsewhere. Less
ice means more heat absorption as the darker water absorbs the
sun’s energy. Arctic summer ice could totally disappear, leaving
wildlife habitats, such as the polar bears, deteriorating or
destroyed.
Eastern Canada
Important species of trees, including the sugar maple, Canada’s
national symbol, will be forced to move northwards, which could
cause problems if the trees cannot adapt. Canadian fisheries will
also struggle, which could be the final straw for the already
endangered Atlantic salmon.
These are just examples of three regions and the effects of a
2°C (3.6°F) rise in temperature. Of course, many other regions
would also suffer similar consequences.
According to the Stern Review on the Economics of Climate Change,
some climate models suggest that a global 2°C (3.6°F) rise above preindustrial
levels would mean that there is potential for the Greenland
ice sheet to begin melting irreversibly, a rising risk of the collapse of
the West Antarctic ice sheet, and a rising risk of the collapse of the
ocean thermohaline circulation.
If temperatures rose more than 5°C (9°F), which is possible if
emissions continue to grow, and positive feedback mechanisms
kick in, such as the release of CO2 from carbon sinks and methane
from permafrost, then the rise in temperatures would be equivalent
to the amount of warming that took place between the end of
the last Ice Age and today.
Such a rise in temperature would be far outside human
experience. A very sobering thought!
The Earth, like a sick human being, is already beginning to
show the effects of higher temperatures. A 2°C (3.6°F) global
temperature rise appears to be the limit recognised as causing
catastrophic climate change.
Staying below 2°C (3.6°F) requires CO2 levels to be stabilised
at 400 ppmv, and this appears unlikely as CO2 levels are already
at 395 ppmv and increasing annually. Greenhouse gas levels are
already at 430 ppm CO2e, ( 2008 level ) and rising at 2.5 ppm CO2e annually.
If this continues, the Earth may well be 2–5°C (3.6–9°F) warmer
by 2050, when greenhouse gas levels would reach about 550 ppm
CO2e.
It seems the only answer will be for all nations and all
individuals to do their bit as far as possible to prevent, or at least
reduce, greenhouse gas emissions. The science appears clear. While
it may not be possible to prevent a 2°C (3.6°F) temperature rise,
it seems everything must be done to prevent rises over and above
this level, and the window of opportunity to do so is rapidly
disappearing.
Key points
➢ Earth’s global mean surface temperature has
increased by 0.74°C (1.33°F) over a hundred-year
period, 1906–2006.
➢ Temperatures in the Arctic however have increased
by about 5°C (9°F) over a similar period.
➢ If greenhouse gases could be halted at present
levels, the Earth would still warm by about 1–3°C
(1.8–5.4°F) above pre-industrial levels (possibly
2.26°C more than present).
➢ The last time Earth was 2–3°C (3.6–5.5°F) higher
than present was 3,000,000 years ago, when sea
levels may have been twenty-five metres (eighty
feet) higher than present.
➢ The warmest year since 1880 was 2005 and 2010, virtually
on a par with 1998, when temperatures were
boosted by an exceptional El Niño year, while 2007
has become Earth’s second warmest year jointly
with 1998.
