Much of the flattening in global temperature rises seen over the last decade can be attributed to a reduction in the concentration of water vapour in the lower stratosphere, according to a group of US climate scientists. Conversely, it is likely that a significant part of the warming observed during the 1990s was due to a rise in such concentrations. The inability of current climate models to incorporate these effects represents a significant weakness in those models, say the researchers.

Global average temperatures on the surface of the Earth have increased by about 0.13° per decade over the last half century. In its 2007 assessment of global warming, the Intergovernmental Panel on Climate Change (IPCC) concluded that it is very likely that most of this rise is due to the accumulation of manmade greenhouse gases, such as carbon dioxide and methane, in the atmosphere. However, water vapour, which is responsible for some 60% of the overall greenhouse effect (needed for human existence), also contributes to global warming via feedback effects.

These effects are relatively well understood in the lowest level of the atmosphere, the troposphere, where increased warming leads to greater evaporation, causing more water vapour and so further warming, although this is offset to some extent through the formation of clouds that reflect incoming sunlight back into space. However, the climatic effects of water vapour in the layer immediately above the troposphere, the stratosphere, are not well understood.

Difficult to model

Scientists know that levels of water vapour have been steadily rising in the upper part of the stratosphere owing to the oxidation of methane that has been building up there since the industrial revolution. But understanding what happens lower down is more difficult. Water vapour is transported upwards from the troposphere but that transport is controlled by temperatures in a thin boundary layer in the tropics known as the tropopause. Unfortunately, modelling this process accurately requires a vertical resolution greater than exists in most climate models.

To work out how much global warming these models might be missing by not tracking the fluctuating levels of water vapour in the lower stratosphere, Susan Solomon and colleagues at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Colorado, together with Gian-Kasper Plattner of the University of Bern in Switzerland, took the observed changes in these concentrations, worked out the heating or cooling effect of these changes and then plugged the resulting numbers directly into a model.

Using satellite data showing a 10% drop in concentrations over the last decade, the researchers found that global surface temperatures rose some 25% more slowly than they would have done had water vapour levels remained static – an increase of 0.10° rather than 0.14°. Applying the same approach to data collected by balloon-borne atmospheric instruments, showing a rise in water-vapour concentrations between 1980 and 2000, the group found that global warming in the 1990s had been enhanced by about 30%.

Long-term negative feedback?

Solomon and colleagues say they don’t know what is causing these changes in concentration. A possible correlation with tropical sea-surface temperatures suggests that levels of stratospheric water vapour might respond systematically to global warming, providing a long-term negative feedback to such warming (since warmer seas in the tropics reduce the transfer of water vapour from the troposphere to the stratosphere). But it is also possible that such concentration changes only take place over a period of a decade and therefore do not provide a long-term check on climate change.

Indeed, Gavin Schmidt, a climate scientist at NASA’s Goddard Institute for Space Studies in New York, points out that the El Niño event of 1997–1998 might have moistened the lower stratosphere more than usual, with the result that this part of the atmosphere has been drying ever since. Alternatively, he says, the cause might be increases in aerosol emissions in Asia, since these have affected temperatures in the tropics.

No matter what the origin is, however, Karen Rosenlof, a member of Solomon’s team, says it is now clear that stratospheric water vapour has a significant effect on global warming and that models’ inability to take this effect into account is a significant failing. “Given the calculated 25% drop in decadal warming,” she points out, “you could say that these models are only 75% right.” But she maintains that this result does not mean that the IPCC is barking up the wrong tree. “It doesn’t change the conclusion that global warming is manmade,” she says.

The research has been published online in Science.

The number of very intense hurricanes striking the east coast of North America and the Caribbean could increase over the next century if ocean temperatures continue to rise. The total number of storms, however, is set to fall. That is according to researchers in the US who have applied a popular model for forecasting cyclone activity to a series of climate projections made by the Intergovernmental Panel for Climate Change (IPCC).

Hurricane Katrina, which struck the southern US including New Orleans in 2005, is an example of the sort of havoc that can be wreaked when North Atlantic cyclones make landfalls. Katrina was a category 5 hurricane, the most intense grade, where sustained wind speeds were as strong as 280 kilometres per hour. Since the 1980s there has been an increase in the number of category 4 and 5 hurricanes forming over the North Atlantic.

Meanwhile, several modelling groups have predicted that rising sea temperatures in the North Atlantic will lead – perhaps counter-intuitively – to a reduction in hurricane activity over the North Atlantic over the course of the 21st century. These climate models, however, fail to provide predict the nature of individual storms because they lack the spatial resolution to determine hurricane intensity. In this latest research Morris Bender and his colleagues at the National Oceanic and Atmospheric Administration (NOAA) have addressed this lack of data in a three-step process.

Seeking the eye of the storm

To begin with, Bender and colleagues predict the effect of rising sea temperatures on the large-scale flow of hurricanes over the Atlantic, with a resolution of about 200 km. They use data based on the average of 18 global models that contributed to the latest scientific report of the IPCC. In the second stage, the researchers feed this information into a regional model of atmosphere over the Atlantic, known as the ZETAC model. At this point, however, they have a spatial resolution of about 18 km, which can only produce storms of the lowest intensity – categories 1 and 2.

The key comes in the final stage when Bender’s team feeds the ZETAC data into a hurricane model that has been used by the US navy since 1995 to predict storms. In this way the researchers are able to resolve the intense winds that circulate close to the eye of the storm in type 4 and 5 hurricanes, made possible by a spatial resolution of 8 km. Called the GFDL Hurricane model, it predicts nearly a doubling of category 4 and 5 hurricanes by the end of the 21st century.

Bender admits that he did not expect quite such an increase, but he is not surprised that rising sea temperatures may have dramatic effects. “We may have the broad predictions for climate but the specific impacts and their locations are far from a done deal,” he tells physicsworld.com.

This research is published in Science.

Researchers in Switzerland and Germany have analysed data stretching back 1000 years to get the best estimate yet of how changes in global temperature affect the biosphere’s ability to soak up carbon dioxide. The team found that this feedback coefficient is about five times smaller than previously expected – which suggests that the amplification of manmade global warming by carbon-cycle feedback will be less than previously thought. Furthermore, the reduced uncertainty of this latest result could lead to better predictions of climate change caused by increasing amounts of carbon dioxide in the atmosphere.

To understand climate change, scientists need to know how changes in global temperature affect the amount of carbon dioxide in the atmosphere. Rising temperatures could, for example, turn a green landscape into a desert, which would reduce that region’s ability to absorb carbon dioxide. Conversely, a warmer climate could lengthen the growing season in mid and high latitudes, increasing the absorption of carbon dioxide in these places. Changes in temperature could also affect the amount of carbon dioxide produced by the vast numbers of micro-organisms in soil.

The overall effect of this “feedback” is expected to be positive – higher temperatures leads to less carbon dioxide being absorbed, which means more of the gas in the atmosphere, which in turn makes the climate even warmer. However, scientists have struggled to get a precise value for the feedback coefficient – a process that involves studying historical carbon dioxide and temperature data.

Large uncertainty

The best estimate had been that a rise in the mean global temperature of one degree boosts the carbon dioxide concentration by about 40 parts per million by volume (40 ppmv/°C) – but this could be off by 30 ppmv/°C or more. Such a large uncertainty makes the prediction of future carbon dioxide levels – and therefore future temperatures – all the more difficult. Indeed, 40% of the uncertainty in such predictions can be attributed to carbon dioxide feedback.

Now, David Frank and colleagues at the Swiss Federal Research Institute in Birmensdorf, the University of Bern and the Gutenberg University in Mainz have performed the most comprehensive analysis of carbon dioxide and temperature data yet. The team studied the period 1050–1800 AD, when manmade emissions were small enough to be ignored. Carbon dioxide levels were determined from three Antarctic ice cores. Average temperatures in the northern hemisphere were derived from nine different “proxy reconstructions” of temperature – average temperatures derived mostly from tree rings and the isotopic content of ice cores.

Two distinct periods

Frank and colleagues conclude that the feedback coefficient is probably about 2–21 ppmv/°C, with 8 ppmv/°C being the median value. The team also found that the coefficient appears to be significantly different in the periods 1050–1549 and 1550–1800 – when it was about 4 and 16 ppmv/°C respectively. The former era corresponds roughly to the “medieval warm period” and 1550–1800 to the “little ice age” – which experienced different patterns of global temperature and precipitation. As a result, Frank and colleagues believe that the shift from one period to the next could have decreased the carbon storage capabilities of some parts of the globe.

The research excludes the previously accepted value of 40 ppmv/°C with a confidence of 95% and provides further evidence that the coefficient is positive rather than negative. The latter is important because it suggests that the biosphere will not be able to soak up manmade emissions of carbon dioxide, which are believed to contribute to global warming.

The work is described in Nature 463 527.