Thursday 29 August 2013

Climate change

It holds enough water to raise sea levels 50 metres, but East Antarctica ice sheet is even more unstable than we thought
The massive ice sheet of East Antarctica, which is up to 4 kilometres thick and holds enough frozen water to raise sea levels by more than 50 metres, is more unstable than previous studies have suggested, scientists said.

27 August, 2013

Declassified images from spy satellites going back 50 years have revealed that the coastal glaciers and floating sea ice of Antarctica are more susceptible to air and sea temperatures than previously supposed, the researchers found.

The images, which cover thousands of miles of East Antarctic's coastline and include measurements of 175 glaciers, show there is rapid and synchronised melting and freezing when local temperatures increase or fall, according to the study published in Nature.

"We know that these large glaciers undergo cycles of advance and retreat that are triggered by large icebergs breaking off at the terminus, but this can happen independently from climate change," said Chris Stokes of Durham University, who led the study.

"It was a big surprise therefore to see rapid and synchronous changes in advance and retreat, but it made perfect sense when we looked at the climate and sea-ice data. When it was warm and the sea-ice decreased, most glaciers retreated, but when it was cooler and the sea ice increased, the glaciers advanced," Dr Stokes said

This satellite image from 2000 shows several major East Antarctic outlet glaciers flowing through the Southern Cross Mountains This satellite image from 2000 shows several major East Antarctic outlet glaciers flowing through the Southern Cross Mountains

Melting sea ice and glaciers could affect the speed at which the ice sheet on the land melts and falls into the sea. "In many ways, these measurements of terminus change are like canaries in a mine - they don't give us all the information we would like, but they are worth taking notice of," he said.

Climate scientists have focussed on the ice sheets of the West Antarctic and Greenland because these were thought to be the less stable then the much bigger ice sheet of East Antarctica, which sits on a mountain range and is much colder than other ice sheets.

Dr Stokes said: "If the climate is going to warm in the future, our study shows that large parts of the margins of the East Antarctic Ice Sheet are vulnerable to the kinds of changes that are worrying us in Greenland and West Antarctica - acceleration, thinning and retreat."

He added: "We need to monitor their behaviour more closely and maybe reassess our rather conservative predictions of future ice sheet dynamics in East Antarctica."

How the Pacific has paused global warming on hold (but not for long)
Although temperatures on Earth are higher than ever, the ocean is playing a major role in absorbing excess heat, a new study suggests

27 August, 2013

Changes in the flow of heat between the atmosphere and the Pacific Ocean could help to explain the recent "pause" in global warming that has seen a fall in the rate at which global surface temperatures has risen over the past 15 years or so, a study has suggested.

The current global warming hiatus, where the increase in global temperatures has levelled off, can be explained at least in part by natural changes to a cold Pacific Ocean current called La Nina which may have helped to absorb excess heat from the atmosphere, scientists said.

It is further evidence that the deep ocean may be playing a major role in helping to dampen down temperature rises at the surface. A previous study for instance found that the heat being absorbed by the deep ocean is equivalent to the power generated by 150 billion electric kettles.

Although global average temperatures are now higher than they have ever been since modern records began, they have not increased as fast over the past 10 or 15 years as some climate models have predicted, leading climate "sceptics" to claim that global warming has stopped.

Climate scientists have countered by saying that the last decades was still warmer than any previous decade, with 12 of the 14 hottest years on record occurring since 2000, and that periods of natural variability, with temperatures falling temporarily, are always to be expected.

Calculations suggest that the overall heat balance of the Earth is showing a positive trend, with more solar heat coming in than thermal energy that is lost into space. Climate researchers have argued that the deep parts of the ocean are likely to be absorbing this extra heat, rather than it accumulating at the Earth's surface.

The latest study by Yu Kosaka and Shang-Ping Xie of the Scripps Institution of Oceanography in San Diego supports this idea by a theoretical study of the cold La Nina current in the eastern Pacific which they found is capable of taking up huge amounts of heat from the atmosphere over a period of decades.

"Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Nina-like decadal cooling," the researchers said in the journal Nature.

"Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase," they said.

In their computer modelling experiment, the two researchers altered the amount of heat flowing between the east Pacific Ocean and the atmosphere so that the sea surface temperatures in the computer model were forced to agree with the actual observations.

"This results in the realistic simulation of the recent surface warming slowdown globally and some unusual weather patterns such as drought experienced in the southern United States," said Richard Allan, a climate scientist at Reading University, who was not involved with the research.

"This new study adds further evidence that the recent slowdown in the rate of global warming at the Earth's surface is explained by natural fluctuations in the ocean and is therefore likely to be a temporary respite from warming in response to rising concentrations of greenhouse gases," Dr Allan said.

"This is important since it adds to a great body of evidence in continuing to confirm the realism of projected dangerous warming in the future due to human activities such as burning fossil fuels," he said.

But bear this in mind

New Research Confirms Global Warming Has Accelerated

25 March, 2013

A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth, and Källén (2013).  There are several important conclusions which can be drawn from this paper.

Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.

As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.

Some recent studies have concluded based on the slowed global surface warming over the past decade that the sensitivity of the climate to the increased greenhouse effect is somewhat lower than the IPCC best estimate.  Those studies are fundamentally flawed because they do not account for the warming of the deep oceans.

The slowed surface air warming over the past decade has lulled many people into a false and unwarranted sense of security.

The main results of the study are illustrated in its Figure 1.

Figure 1: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.

The Data

In this paper, the authors used ocean heat content data from the European Centre for Medium-Range Weather Forecasts' Ocean Reanalysis System 4 (ORAS4).  A ‘reanalysis’ is a climate or weather model simulation of the past that incorporates data from historical observations.  In the case of ORAS4, this includes ocean temperature measurements from bathythermographs and the Argo buoys, and other types of data like sea level and surface temperatures.  The ORAS4 data span from 1958 to the present, and have a high 1°x1° horizontal resolution, as well as 42 vertical layers.  As the authors describe the data set,
"ORAS4 has been produced by combining, every 10 days, the output of an ocean model forced by atmospheric reanalysis fluxes and quality controlled ocean observations."

Accelerated Global Warming

As illustrated in Figure 1 above, the study divides ocean warming into three layers for comparison – the uppermost 300 meters (grey), 700 meters (blue), and the full ocean depth (violet).  After each of the Mt. Agung, Chichón, and Pinatubo volcanic eruptions (which cause short-term cooling by blocking sunlight), a distinct ocean cooling event is observed in the data.  Additionally, after the very strong El Niño event of 1998, a cooling of the upper 300 and 700 meters of oceans is visible as a result of heat being transfered from the surface ocean to the atmosphere.
One of the clearest features in Figure 1 is the rapid warming of the oceans over the past decade.  As we have previously discussed, the warming of the shallower oceans has slowed since around 2003, which certain climate contrarians have cherrypicked to try and argue that global warming has slowed.  However, more heat accumulated in the deeper oceans below 700 meters during this period.  The authors describe the ocean warming since 1999 as,
"the most sustained warming trend in this record of OHC.  Indeed, recent warming rates of the waters below 700m appear to be unprecedented."
Their results in this respect are very similar the main conclusion of Nuccitelli et al. (2012), in which we noted that recently, warming of the oceans below 700 meters accounts for about 30% of overall ocean and global warming.  Likewise, this new study concludes,
"In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend."
The warming of the oceans below 700 meters has also been identified by Levitus et al. (2012) and Von Schuckmann & Le Traon (2011), for example.

Some 'Missing Heat' Found

Kevin Trenberth's past comments about 'missing heat' drew considerable attention.  The phrase refers to the fact that the heat accumulation on Earth since about 2004 (e.g. from warming oceans, air, and land, and melting ice) that instruments were able to measure could not account for the amount of global heat accumulation we expected to see, based on the energy imbalance caused by the increased greenhouse effect, as measured by satellites at the top of the Earth's atmosphere.
These new estimates of deeper ocean heat content go a long way towards resolving that 'missing heat' mystery.  There is still some discrepancy remaining, which could be due to errors in the satellite measurements, the ocean heat content measurements, or both.  But the discrepancy is now significantly smaller, and will be addressed in further detail in a follow-up paper by these scientists.
So what's causing this transfer of heat to the deeper ocean layers?  The authors suggest that it is a result of changes in winds related to the negative phase of the Pacific Decadal Oscillation and more frequent La Niña events.
Good News for Climate Sensitivity?  Probably Not

Recently there have been some studies and comments by a few climate scientists that based on the slowed global surface warming over the past decade, estimates of the Earth's overall equilibrium climate sensitivity (the total amount of global surface warming in response to the increased greenhouse effect from a doubling of atmospheric CO2, including amplifying and dampening feedbacks) may be a bit too high.  However, as we previously discussed, these studies and comments tend to neglect the warming of the deep oceans below 700 meters.
Does the warming of the deep ocean support these arguments for lower equilibrium climate sensitivity?  Probably not, as Trenberth explained (via personal communication),
"it contributes to the overall warming of the deep ocean that has to occur for the system to equilibrate.  It speeds that process up.  It means less short term warming at the surface but at the expense of a greater earlier long-term warming, and faster sea level rise."
So the slowed warming at the surface is only temporary, and consistent with the 'hiatus decades' described by Meehl et al. (2011).  The global warming end result will be the same, but the pattern of surface warming over time may be different than we expect.
The real problem is that in the meantime, we have allowed the temporarily slowed surface warming to lull us into a false sense of security, with many people wrongly believing global warming has paused when in reality it has accelerated.
Global Warming Wake Up Call

Perhaps the most important result of this paper is the confirmation that while many people wrongly believe global warming has stalled over the past 10–15 years, in reality that period is "the most sustained warming trend" in the past half century.  Global warming has not paused, it has accelerated.
The paper is also a significant step in resolving the 'missing heat' issue, and is a good illustration why arguments for somewhat lower climate sensitivity are fundamentally flawed if they fail to account for the warming of the oceans below 700 meters.
Most importantly, everybody (climate scientists and contrarians included) must learn to stop equating surface and shallow ocean warming with global warming.  In fact, as Roger Pielke Sr. has pointed out, "ocean heat content change [is] the most appropriate metric to diagnose global warming."  While he has focused on the shallow oceans, actually we need to measure global warming by accounting for all changes in global heat content, including the deeper oceans.  Otherwise we can easily fool ourselves into underestimating the danger of the climate problem we face.

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