World's oceans are getting warmer, faster
- 25 Mar 2013, 13:00
- Roz Pidcock
When looking at how global temperatures have changed, it's easy
to focus on the atmosphere. But as a new paper shows, we should be
looking at the oceans too - and the deep ocean in particular. Over
the last half century, new data shows the oceans have warmed
substantially - accelerating in the last decade.
Since the start of the 20th century, land and sea surface
temperatures have risen by about
0.76 degrees Celsius, mainly as a result of
burning fossil fuels.
But the heat that stays trapped in the atmosphere is only a
small fraction of the sun's energy that hits the earth. Previous
studies show about
90 per cent of the heat is absorbed by oceans.
new study, just published in the journal Geophysical Research
Letters, shows just how much the oceans have warmed in the past 50
years as a result. Notably, it finds the rate of warming has
accelerated since about 2000.
Observing the oceans
The scientists combined historical ocean temperature data
collected in a number of different ways: remotely-sensed data from
satellites, direct measurements made by instruments deployed from
ships and free-drifting vehicles known as ARGO floats.
The team fed the measurements - spanning the period 1958 through
to 2009 - into an ocean and atmospheric circulation
model from the European Centre for Medium-Range Weather
Forecasts (ECMWF). The model
simulates the historical observations and produces an estimate of
warming at different depths in the ocean every 10 days.
Long term warming
The purple lines in the graph below show how the heat content of
the whole ocean has changed over the past five decades. The blue
lines represent the top 700 m and the grey lines are just top 300
Balmaseda et al., (2013)
Each of the five lines in each set represents a single model
run, each with slight differences in the starting conditions or in
details of the model. This is one way scientists account for
uncertainty in the climate system or in the measurements
The data show a general warming trend in the oceans over the
last five decades - but there are a couple of important features to
Short, sharp cooling events
There are a few times in the last half century when the whole
ocean has lost a lot of heat to the atmosphere quite suddenly after
a major volcano eruption, such as Pinatubo in 1992.
periodic fluctuation in the climate known as the El Nino
Southern Oscillation (ENSO) also affects the
amount of heat stored in the ocean. In El Nino years, notably 1997
to 1998, the ocean releases a lot of that heat into the atmosphere
- causing atmospheric temperatures to warm.
Previous studies that have looked at how the ocean's uptake of
heat has changed over time have only been able to "hint at" the
short, sharp cooling events that have punctuated the long term
warming record, co-author Professor Kevin Trenberth told Carbon
Probably the most important feature for analysing recent global
temperature is that the rate of ocean warming has accelerated
sharply since about 2000 - as the rise in the purple line in the
graph above shows.
But the warming isn't evenly distributed throughout the ocean.
Warming in the top 300 m of the ocean - shown in grey - has slowed
slightly since about 2000.
Only the deeper ocean shows accelerated warming, at a rate
unprecedented in the last 50 years. This is pretty clear in the
blue line - which is the top 700 m - but the pattern is strongest
in the model data for the whole ocean, which extends to below 5000
The scientists calculated that warming of the ocean below about
700 m now accounts for about 30 per cent of the total heat entering
the oceans. The new paper also suggests a reason why more heat is
being transported out of the surface ocean. Trenberth tells us:
"The cause of the change is the change
in winds, especially in the Pacific Ocean where the subtropical
trade winds have become noticeably stronger, thereby increasing the
subtropical overturning in the ocean and providing a mechanism for
heat to be carried down into the ocean."
The new study helps shed light on an issue known as the '
missing heat' problem. Scientists can
calculate how much the planet should be warming based on the
amount of solar radiation hitting earth and the level of greenhouse
gases in the atmosphere. But since 2004, measurements of heat
uptake by the land, the surface oceans, the atmosphere and melting
ice have fallen short of the expected warming.
Together with the fact that global atmospheric temperatures have
risen slower over the past decade compared to previous decades,
this has led to
claims by some that global warming has paused or even
The new study doesn't completely close the gap - possibly
because of some errors remaining in the measurements, Trenberth
tells us. But it does show where a lot of the 'missing heat' is
going - into the deep ocean.
Trenberth tells us:
"[The new study] also means that the
current hiatus in surface warming is a transient and global warming
has not gone away."
Global warming isn't all about atmospheric temperatures. The new
study highlights the importance of looking at the surface of the
ocean right down to the deepest depths when trying to understand
how sensitive the atmosphere is to increasing greenhouse gases - a
concept known as the climate sensitivity. The role of the oceans in
climate change - particularly below 700 m - can't be
Balmaseda et al., (2013) Distinctive climate signals in
reanalysis of global ocean heat content. Geophysical
Research Letters, doi:
25/03 14:50 pm Modified to add the correct maximum depth of
the model, over 5000 metres.