Diving deep into ocean data uncovers ‘missing heat’ treasure

A new ocean reanalysis called ORAS4, here showing the difference between September 2012 sea temperatures and the average for 1989-2009 (not part of the latest study), has helped show that extra heat trapped in the atmosphere by CO2 humans are emitting is buried in the deep ocean. Credit: ECMWF

A newly-made picture of ocean history has backed a theory that the missing piece of a climate puzzle at the edge of space lies deep in Earth’s waters. The puzzle comes because the amount of heat energy our planet has absorbed should have warmed it more than it seems to have done. But now, using an ocean reanalysis assembled from data gathered from many sources, UK and US researchers have shown especially strong recent warming in oceans below 700m. “We have found some energy buried at depths,” Kevin Trenberth from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. “We also have a plausible explanation for it related to changes in winds.”

In 2010, Kevin went public over his worries about a budget that didn’t balance. But rather than money, that budget tallies heat energy from the Sun entering the top of the atmosphere against energy the Earth radiates back out into space. Satellite measurements show more energy coming in than leaving, which is what causes global warming. But Kevin noticed that existing measurements showed the world hadn’t warmed as much since 2003 as this budget would suggest.

With over nine-tenths of the surplus energy coming into the Earth going into the sea, the deep ocean has always looked the likeliest hiding place for the missing heat. However, temperature data from those depths is scarce, making the theory hard to prove. Yet, in the years since Kevin pointed out the problem, scientists have gathered some clues to back that explanation. For example, some used a model that includes the complex links between the atmosphere, land, oceans, and sea ice to run five simulations of the 21^st century. They found warming slowdowns on the Earth’s surface similar to what has happened in the 2000s, with the heat going into the deep oceans. But even this just underlined the importance of using measurements to see the effect directly.

Space agencies pinpoint polar ice sheet damage

The midnight sun casts a golden glow on an iceberg and its reflection in Disko Bay, Greenland, where ice sheet mass loss was five times higher in 2011 than it was in 1992. Much of Greenland’s annual mass loss occurs through ‘calving’ of icebergs such as this. Credit: Ian Joughin.

47 scientists from 26 key laboratories across the world. 10 satellite missions flown over a period of 20 years, whose data adds up to 51 years’ worth. This giant effort looks to have squashed stubborn uncertainty surrounding one key climate question: How quickly are ice sheets resting on land masses at the North and South Poles shrinking? The international team has now found that Greenland’s mass loss is five times as fast as it was in 1992. Overall loss rates in Antarctica are roughly constant in this period, though the east of the continent is actually gaining ice. Over the past 20 years, the polar ice sheets have added 11 mm to sea level rise across the world, one-fifth of the total rise seen in that time.

“Our new estimates are the most reliable to date and they provide the clearest evidence yet of polar ice sheet losses,” said Andrew Shepherd from the University of Leeds, UK, co-leader of the project. “They also end 20 years of uncertainty concerning changes in the mass of the Antarctic and Greenland ice sheets and they’re intended to become the benchmark dataset for climate scientists to use from now on.”

Until the early 1990s, climate researchers expected that mass lost by ice sheets in Greenland as the planet warmed would be balanced by that gained by Antarctica. But measurements showed that both melting and ‘calving’ of icebergs could be speeding up at both poles. This meant the UN’s Intergovernmental Panel on Climate Change (IPCC) couldn’t put an upper limit on what ice sheets might add to sea levels in its last major report on global warming in 2007. And the overall picture has been confused, as efforts to measure whether ice sheets are shrinking or growing have given differing results. Since 1998, there have been 29 different estimates of changes in ice sheet mass. “Taken all of the past studies together, the recent global sea level contribution due to Antarctica and Greenland may have been anywhere between a 2 mm per year rise and a 0.4 mm per year fall,” Andrew told a press conference yesterday. At a workshop in 2010, the IPCC said it was concerned that no further progress would be made by its next report, due in 2014.

New models still give Arctic summer ice 30 years

A thin sheet of sea ice reflects the rising sun off the east coast of Greenland on Apr. 14, 2012, with thicker sea ice and icebergs in the background. On average, the most up to date climate models that accurately simulate recent Arctic ice melting predict a nearly ice-free September by 2035. Credit: NASA/Jefferson Beck

Predictions from a collection of the latest climate models on average say that ice will be nearly gone from the Arctic by the 2030s. But when you don’t include man-made – or ‘anthropogenic’ – CO2 emissions’ ‘forcing’ effect, those models show a much icier picture.  “This clearly shows that if you don’t consider anthropogenic forcing, the ice won’t decline that fast,” said Muyin Wang from the University of Washington. “It should be oscillating around a much higher level.”

These findings echo some that Muyin and her Seattle colleague James Overland, from the US National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory, made in 2009. Then, James and Muyin used climate models that formed the basis for the Intergovernmental Panel on Climate Change’s fourth assessment report, which was published in 2007. “Because of this report’s success a lot more modelling groups around the world started doing simulations,” Muyin told me. Scientists are now bringing their improved old models together with new ones in a project to compare them. Having found the old models bad at reproducing measured shrinkage of Arctic ice at the end of the 20^th century, James and Muyin wanted to see if the new and improved ones could do any better.

It’s important to be able to reproduce real data to be confident in models’ predictions, Muyin said. “If you are interviewing someone for a job, you look at their resumé, to see if they did a good job in the past,” she explained. “Then you know that they can do the job going forward. It’s a similar idea here, if models can simulate the past climate, then they’re the models we want to use in the projection.”

In a paper published in the scientific journal Geophysical Research Letters on Tuesday, they started from 32 different models, and compared them with satellite data on sea ice coverage. Overall, their resumés were slightly better than the older models: For the period from 1981-2005, the average of all these models was near the ice coverage actually seen, whereas the older models had overestimated the values.  But the highest and lowest estimates in both groups were still very similar.

Beefing down farming could cut carbon

University of Exeter’s Tom Powell. Credit: University of Exeter

If people like me in the developed world eat less steak, it could free up room for plants to reduce CO2 levels in the air driving climate change. That’s one forecast that has come from Tom Powell and Tim Lenton at the University of Exeter, who have studied how much space we’ll need for food in the future. “The impact on the environment of trying to produce the food demanded by the world’s population in the future could be disastrous, unless we make the production system much more efficient,” Tom told Simple Climate. “By far the easiest way to do this would be to cut meat eating, especially beef. Meat is likely to get more expensive as the resources needed to produce it become limited, and its environmental impact grows. Small changes to our diets and the ways we produce food have the potential to make what is currently a very environmentally damaging system a much more positive one.”

When we’re buying food, its climate impact may not be immediately obvious. But plants use the sun’s energy to take CO2 out of the atmosphere as they grow, storing that energy and CO2 in their bodies for a comparatively short time. “We can’t escape the links between our energy use, whether it’s for diet, industry or transport, and the carbon cycle,” Tom underlined. “Unfortunately, the carbon cycle also controls a sort of global thermostat, with the amount in the atmosphere as CO2 or methane influencing the climate.”

Tim and Tom noted that as the number of us on the planet grows, and we get wealthier, we are demanding more energy, both as food and fuel. “This is having damaging effects on ecosystems, and even on the world’s climate as the population grows toward 9.5 billion people all aspiring to a western lifestyle,” Tom underlined. As people get richer they also eat more meat – but meat production is hugely inefficient. Only about three or four parts in 100 of the feed energy livestock eat becomes food, with the rest lost as manure, heat, methane and slaughter by-products. Today, meat consumption provides one-sixth of the energy people across the world get from their food on average. However, people in rich countries eat much more meat, getting almost one third of their energy from it.

Thick ice decline could advance watery Arctic summers

NASA's shipborne ICESCAPE mission cuts a path through multiyear Arctic ice last year. This thicker form of ice is declining fastest, NASA's Joey Comiso has shown in a separate study. Credit: NASA/Kathryn Hansen

The oldest and thickest ice in the Arctic is vanishing the fastest, data studied by NASA scientist Joey Comiso and published last month have shown. “This is alarming since it is usually the thick component that would survive the long summer melt period,” Joey told Simple Climate. “Since the thick component is declining more rapidly, the Arctic summer ice cover is more vulnerable to further decline. Assuming that the surface temperature continues to warm up as it has in the last several decades, this makes it more likely that we will have very little or no sea ice cover in the summer sooner than we previously expected.” And when the Arctic is ice-free in summer, dramatic environmental changes could follow that would speed warming further and limit the supply of fish for food.

Having long studied Arctic sea ice cover, Joey previously showed that 2007′s record smallest summer area was around one quarter smaller than the previous minimum in 2005. That “has been regarded as the event that could trigger an irreversible change in the Arctic sea ice cover”, Joey wrote in this latest research paper in the Journal of Climate. But after that low the area of thickest ice that can survive the summer melts, known as perennial ice, recovered slightly before dipping again this winter.

Intrigued by that recovery, Joey wanted to understand it. He therefore turned to data collected by tools called microwave radiometers that have been flying over the Arctic on satellites since 1979. These can collect information on the ability of different objects to emit microwave energy, or their microwave emissivity. Salt content, or salinity, influences this emissivity. As sea ice is initially around one-third as saline as sea water, microwave emissivity can be used to tell one from the other. It can also separate multiyear ice, which has survived at least two summer melt seasons, from thinner second year ice that has only survived one summer.

Fjord beds show climate role in glacier mass loss

Sun setting over Sermilik Fjord into which Helheim Glacier calves large amounts of icebergs each summer. Credit: Camilla S. Andresen.

A recent surge of icebergs produced by Helheim Glacier in Greenland has only been exceeded once in more than a century. That’s according to Camilla Andresen from the Geological Survey of Denmark and Greenland (GEUS) and her colleagues, who have reconstructed a record of iceberg production back to 1890. Their work helps add to our currently limited knowledge of what causes glaciers to grow and shrink, pointing to water temperature and an Atlantic climate cycle as being largely responsible at Helheim. Andresen underlined that this establishes a role for climate in iceberg formation or “calving” from glaciers – one that suggests that it will increase as temperatures rise. “With the link we have found to climate I find it very likely that they will respond to future warming and continue to lose mass,” she told Simple Climate.

Increased calving in the early 2000s helped rapidly shrink the Greenland Ice sheet – but scientists were unclear why, and whether the event was unusual. “Many scientists believe that this was associated with increased inflow of warm Atlantic Ocean waters deep into the fjords to where the glaciers terminate,” Andresen explained. “Changes in the volume and/or temperature of these water masses would affect underwater melting of the glacier front and iceberg calving rates. We know these processes are important for understanding the future behaviour of the inland ice sheet.”

Tension simmers over climate link to plant growth

A misty canopy at dawn in the Amazon forest, where calculations of plant growth from satellite measurements that differ from direct measurements have come under criticism. Image courtesy of Peter van der Steen

A surprise finding that plants are growing less quickly when, with current temperatures and CO2 levels, we might expect the opposite has come under fire from two independent groups of scientists. In August 2010, Maosheng Zhao and Steven Running from the University of Montana, Missoula showed that a measure of plant growth speed had slowed slightly since 2000. That’s even though it had accelerated in the 1980s and 1990s, a situation that fits the higher levels of CO2 in the atmosphere available to help plants grow through photosynthesis. But now, a year later, scientists from the US and Brazil have complained that this does not match what they’ve seen directly in Amazonian forests. Meanwhile, an Australian scientist adds to these objections with claims that Zhao and Running have over-estimated the effect that temperature has had on growth rates.

The disagreement focuses on how Zhao and Running calculated the measure of plant growth they use – net primary productivity (NPP) – from satellite data. “Measuring growth of a single tree is easy, however, at the global level, for billions of trees and plants, measurement of growth is only possible with data from satellites,” Zhao told Simple Climate. “Our model uses vegetation greenness information observed from satellites and daily global weather data to calculate vegetation growth of each kilometre over 110 million square kilometres of vegetated land surface.” The downside in this approach is that the view of Earth’s surface is often blocked, for instance by clouds and smoke. When that happens, the Montana researchers use the data from before and after the days the satellite can’t see the surface to fill in the missing measurements. They can then calculate NPP from those vegetation greenness measurements.  

Fire amid the ice kindles global and local worries

The Anaktuvuk River fire burning in August 2007 on the North Slope of the Brooks Range in Alaska. University of Florida ecologist Michelle Mack and a team of scientists including fellow UF ecologist Ted Schuur found the fire released a significant amount of soil-bound carbon into the atmosphere. Credit: Alaska Fire Service

In 2007, the largest Arctic tundra wildfire on record released around 2.1 million tonnes of carbon to the atmosphere, adding to the levels of greenhouse gas CO2. That’s close to how much carbon tundra plant growth across the whole Arctic absorbs in one year, noted Michelle Mack at the University of Florida. With human-caused, or anthropogenic, climate change seemingly causing more fires, further CO2 release may contribute to more warming. Other consequences will also have a big local impact, Mack told Simple Climate. “Fire on this landscape will change many things, and that’s frightening for me because I do think that the increasing fires are driven by anthropogenic climate change,” she said. “That people emitting carbon from cities, factories and automobiles very far to the south are influencing this wilderness area where people still practise subsistence livelihood is disturbing to me.”

For about a decade, Mack has been a regular at the Toolik Field Station on the North Slope of the Brooks Range of mountains in far northern Alaska. After the fire started in July 2007 at Anaktuvuk River, a plume of smoke could be seen drifting through the air from the Toolik Field Station 15 miles to the southeast. “When it started it was characteristic of these tundra fires – very small – just a couple of hectares from a lightning strike,” Mack said. “Normally a fire like that would just go out and there would just be a little blackened spot. It wasn’t until August that the weather conditions were such that the fire blew up and burned a really large area. At that time you could see it from space. People in local villages like Anaktuvik Pass and other coastal villages were getting smoked out. People were miserable.”

Aerosols paint clearer warming slowdown picture

Light from a LIDAR instrument forms a beam in the sky over Boulder, Colo.. NOAA researchers and colleagues used LIDAR data to better understand recent changes in the amounts of tiny particles high in Earth's atmosphere.Credit: CIRES/NOAA

Increased amounts of tiny, airborne solid and liquid particles between 10 and 50 km above the Earth’s surface have slowed recent global warming, US scientists said Thursday. Such mixtures of particles in air, called aerosols, are already known to influence climate, including by reflecting energy from the sun back out into space. Now, Ellsworth Dutton and his colleagues from the US National Oceanic and Atmospheric Administration (NOAA) have identified previously overlooked aerosols, he told Simple Climate. “Additional aerosols that had not been previously known contributed to the planet being slightly cooler than it would have been without those aerosols over the past about a decade,” he said.

From 2002-2009, average global temperatures decreased – although, due to the short length of this period, this trend fails the key scientific test of “statistical significance”. This is despite increased amounts of the greenhouse gas CO2 being emitted by humans burning fossil fuels, which basic climate science predicts should warm the planet. Though Dutton and his colleagues were not seeking to resolve this contradiction, they were interested in how the composition of Earth’s atmosphere varies over periods of months to years. “Several sets of measurements were indicating some small but consistent changes that had not yet been analysed and evaluated,” Dutton said. “Then several of us got our information together to see what it might be telling us.”

Shrinking Arctic ice area is just the tip of the iceberg

Like icebergs, much of the mass of Arctic ice lies under the surface, making studying its thickness important, as well as the area it covers. Credit: Florida State University

A sheer white glacial mountain apparently floating on the sea emerges from the freezing mists. A lone lookout cries “Iceberg!”, stirring the crew into panic, before the stomach-churning sound of ripping metal sends them to the lifeboats. It’s a familiar scene from the TV and movies, and one where the main threat comes because eight-ninths of a typical iceberg lies below the waterline. While sea ice formed from ocean water is much thinner than such glacier ice, its thickness also lies mostly beneath the surface. Consequently, knowing that thickness is “perhaps the most basic measure of how the ice is responding to climate change”, according to the University of Colorado’s James Maslanik. Although it’s possible to track the area that the Arctic ice cap covers from space, knowing the depth it reaches is harder. “While the European Space Agency has just launched Cryosat-2 to measure thickness, the US no longer has a satellite operating that is capable of directly measuring ice thickness from space,” the scientist explained.

To tackle this problem, Maslanik and Colorado colleagues Julienne Stroeve, Charles Fowler, and William Emery have turned to assessing how many summer melt periods the ice is surviving. Maslanik says not only is this closely linked to ice thickness, it also reflects the influence of many climate- and weather-related factors. Ice that survives one summer melt is called “multi-year ice”. In a Geophysical Research Letters paper soon to be published, they find that multi-year ice makes up 45 per cent of the total Arctic ice cover in 2011, down from about 75 per cent in the mid 1980s. The proportion of ice older than five years fell from 50 per cent of all ice that has survived more than one summer to 10 per cent in the same period.

“The work of our Colorado group and other researchers clearly shows extreme decreases in the area of the Arctic Ocean covered by the oldest and thickest sea ice types,” Maslanik told Simple Climate. “This loss has accelerated in recent years, and while we continue to search for factors such as natural variability that could account for the changes, the effects of large-scale warming in the Arctic, including changes in the Arctic Ocean itself, are the most likely drivers for the loss in the old sea ice.”