Taking Earth's temperature: Three Met Office reports examine the warming pause, climate sensitivity and taking a broad view of climate change
- 25 Jul 2013, 12:45
- Roz Pidcock
Much of the debate around climate change taking place in the
media, online and in the world of campaigning focuses on the part
of the planet where humans live - the surface. We are warned about
climate change in terms of temperature rise - two degrees is
'dangerous' - and when temperatures don't rise steadily, some claim
that "global warming has stopped".
saw a piece on the BBC's Sunday Politics which focused on a
showing surface temperature from 1980 to argue global warming
had "plateaued", suggesting that climate scientists were at a loss
to predict or explain it.
With impeccable timing, the Met Office has just released
three useful reports on this very topic, which explore some
reasons why looking only at surface temperatures might mean you're
missing the broad picture.
Taking earth's temperature
three reports, published this week, have a lot of interesting
detail and are worth a read in full.
first looks at what different measurements of the earth's
can tell us about climate change. The
second looks at what's causing surface warming to slow, and the
third looks at whether the slowdown affects projections of
substantial warming by the end of the century.
On surface temperatures the Met Office says:
"Global mean surface temperatures rose
rapidly from the 1970s, but have been relatively flat over the most
recent 15 years to 2013."
A broader look at climate change considers other indicators of
how the climate is changing, the reports say. One pretty clear
indicator is Arctic sea ice. The report shows the following graph
and explains how the amount of ice - particularly in summer - has
continued to decline:
"Over the last decade, while
global surface temperature rise has paused, the Arctic land areas
have continued to warm markedly ... The record low summer Arctic
sea ice extent observed in 2012 represents a 50 per cent reduction
in ice cover compared to the 1980s".
On observations of glaciers, which globally have shrunk every
year for 22 years in a row, the report says:
"[T]he continuing retreat of most of the
world's glaciers is one of the clearest signals of ongoing climate
The report also explains how new measurements from the ARGO network of ocean buoys
are beginning to offer
new insights into how the ocean temperatures are changing. And
that has a lot to do with what's causing surface warming to slow,
it says - the topic of the
Pause for thought
The amount of the sun's energy reaching earth hit a pronounced
low in recent years as part of an 11-year cycle, but the drop is
not enough to account for the surface warming slowdown, the report
When volcanoes erupt they spit reflective particles into the
atmosphere, but the cooling effect drops off after two to three
years, according to the report. No major eruptions recently means
volcanic activity can't explain the slowdown in surface warming
In earth's recent past, other
natural fluctuations in the climate have caused several periods
little surface warming, or even cooling. The Met Office
suggests two such periods lasting about ten years are expected on
average each century - and that in that respect the current
slowdown isn't unusual. However, it does say that a period lasting
more than 20 years is unlikely unless something other than natural
variability is contributing too.
The report suggests that while the land's surface has continued
to warm in the last decade, the slowdown in surface temperature is
at least in part due to how the oceans take up heat.
Data from the ARGO floats traversing the world's oceans indicate
the surface of the oceans hasn't warmed much in the last decade,
while warming below 300m has
increased. That means the total heat absorbed has continued to
rise, shown by the blue line in the graph below.
The report highlights some caveats to this. Ocean measurements
don't exist over a long enough period of time or a large enough
volume of the ocean to prove the theory definitively. For example,
how much the ocean below 700 m is warming is less clear - as there
are fewer measurements there.
But the report says the evidence scientists have so far
indicates heat is going into the deeper ocean instead of staying at
Importantly, the Met Office suggests this redistribution of heat
isn't random but probably caused by changes in
ocean circulation, with the Pacific Ocean playing a key role.
At some point natural cycles will reverse and the oceans will
release that heat, causing surface temperatures to rise again:
"[T]he heat in the ocean is merely being
rearranged; it is effectively 'hidden' from the surface only to
re-emerge at some later date."
Slow changes in ocean circulations are a good reason for looking
at more than just a decade to determine how temperatures are
changing, the report says:
"[V]ery little can be concluded from
ten-year trends with respect to global warming … It is only with
averaging periods of 30 years or longer that climate change can be
Does the hiatus mean we'll see less warming
third report addresses whether the recent period of slower
surface warming should make scientists lower their assessment of
how sensitive the climate is to greenhouse gases - something called
On this, the Met Office says:
"[T]he prospects of substantial global
warming by the end of the century are not materially altered by the
recent pause in global surface warming."
That's largely because even with the slowdown in surface
warming, the average temperature for last decade was still the warmest
on record, as the graph below shows.
But while the amount of warming in the long term doesn't seem
affected, the heat from the oceans might take a while to be
released - which means how much of that warming we're likely to see
in the next few decades might come down a bit. The report says:
"The most likely warming is reduced by
only ten per cent, indicating that the warming that we might
previously have expected by 2050 would be delayed by only a few
Some scientific literature suggesting
possible lower climate sensitivity is being considered by the
Intergovernmental Panel on Climate Change (IPCC), so it will be
interesting to see how the IPCC's assessment compares with the Met
Office's when it is published in September.
Understanding how greenhouse gases are affecting the planet
means understanding more than how surface temperatures are
changing. As the Met Office puts it:
"[C]onsiderations of our climate and its
long-term change must continue to be made on the basis of robust,
holistic assessments - including, but not limited to, global mean
surface temperature - to reflect the fullest possible picture of
our changing climate ."