2008 Likely To Be One Of The Top-ten Warmest Years

ScienceDaily (Jan. 8, 2008) — 2008 is set to be cooler globally than recent years say Met Office and University of East Anglia climate scientists, but is still forecast to be one of the top-ten warmest years.

Each January the Met Office, in conjunction with the University of East Anglia, issues a forecast of the global surface temperature for the coming year. The forecast takes into account known contributing factors, such as El Niño and La Niña, increasing greenhouse gas concentrations, the cooling influences of industrial aerosol particles, solar effects and natural variations of the oceans.

Global temperature for 2008 is expected to be 0.37 °C above the long-term (1961-1990) average of 14.0 °C, the coolest year since 2000, when the value was 0.24 °C.

For 2008, the development of a strong La Niña in the tropical Pacific Ocean will limit the warming trend of the global climate. During La Niña, cold waters upwell to cool large areas of the ocean and land surface temperatures. The forecast includes for the first time a new decadal forecast using a climate model. This indicates that the current La Niña event will weaken only slowly through 2008, disappearing by the end of the year.

Prof. Chris Folland from the Met Office Hadley Centre said: "Phenomena such as El Niño and La Niña have a significant influence on global surface temperature and the current strong La Niña will act to limit temperatures in 2008. However, mean temperature is still expected to be significantly warmer than in 2000, when a similar strength La Niña pegged temperatures to 0.24 °C above the 1961-90 average. Sharply renewed warming is likely once La Niña declines."

These cyclical influences can mask underlying warming trends with Prof. Phil Jones, Director of the Climatic Research Unit, University of East Anglia, saying: "The fact that 2008 is forecast to be cooler than any of the last seven years (and that 2007 did not break the record warmth set on 1998) does not mean that global warming has gone away. What matters is the underlying rate of warming - the period 2001-2007 with an average of 0.44 °C above the 1961-90 average was 0.21 °C warmer than corresponding values for the period 1991-2000."

It is most unlikely that 2008 will be as warm as or warmer than the current warmest year of 1998, which was 0.52 °C above the long-term 1961-1990 average because it was dominated by an extreme El Niño.

Interannual variations of global surface temperature are strongly affected by the warming influences of El Niño events the cooling influences of La Niña events. The year 2007, with a provisionally assessed temperature of 0.41 °C (above long-term average), was colder than forecast. This was due to a much quicker than expected decline of a moderate El Niño that warms the climate, followed by the development of the strong cooling influence of the current La Niña.

The current La Niña event is now the strongest since 1999-2000. The lag between La Niña and the full global surface temperature response means that the cooling effect of La Niña is expected to be a little greater in 2008 than it was during 2007.

Over the eight years, 2000-2007, since the Met Office has issued forecasts of annual global temperature, the mean value of the forecast error was just 0.07 °C.

The Met Office Hadley Centre is the UK's foremost centre for climate change research. Partly funded by Defra (the Department for Environment, Food and Rural Affairs) and the Ministry of Defence.

Adapted from materials provided by Met Office Hadley Centre.

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North Atlantic Warming Tied To Natural Variability

ScienceDaily (Jan. 5, 2008) — A Duke University-led analysis of available records shows that while the North Atlantic Ocean's surface waters warmed in the 50 years between 1950 and 2000, the change was not uniform. In fact, the subpolar regions cooled at the same time that subtropical and tropical waters warmed.

This striking pattern can be explained largely by the influence of a natural and cyclical wind circulation pattern called the North Atlantic Oscillation (NAO), wrote authors of a study published Jan. 3, in Science Express, the online edition of the journal Science.

Winds that power the NAO are driven by atmospheric pressure differences between areas around Iceland and the Azores. "The winds have a tremendous impact on the underlying ocean," said Susan Lozier, a professor of physical oceanography at Duke's Nicholas School of the Environment and Earth Sciences who is the study's first author.

Other studies cited in the Science Express report suggest human-caused global warming may be affecting recent ocean heating trends. But Lozier and her coauthors found their data can't support that view for the North Atlantic. "It is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming," they wrote.

"The take-home message is that the NAO produces strong natural variability," said Lozier in an interview. "The simplistic view of global warming is that everything forward in time will warm uniformly. But this very strong natural variability is superimposed on human-caused warming. So researchers will need to unravel that natural variability to get at the part humans are responsible for."

In research supported by the National Science Foundation in the United States and the Natural Environment Research Council in the United Kingdom, her international team analyzed 50 years of North Atlantic temperature records collected at the National Oceanic Data Center in Washington, D.C.

To piece together the mechanisms involved in the observed changes, their analysis employed an ocean circulation model that predicts how winds, evaporation, precipitation and the exchange of heat with the atmosphere influences the North Atlantic's heat content over time. They also compared those computer predictions to real observations "to test the model's skill," the authors wrote.

Her group's analysis showed that water in the sub-polar ocean --- roughly between 45 degrees North latitude and the Arctic Circle --- became cooler as the water directly exchanged heat with the air above it.

By contrast, NOA-driven winds served to "pile up" sun-warmed waters in parts of the subtropical and tropical North Atlantic south of 45 degrees, Lozier said. That retained and distributed heat at the surface while pushing underlying cooler water further down.

The group's computer model predicted warmer sea surfaces in the tropics and subtropics and colder readings within the sub-polar zone whenever the NAO is in an elevated state of activity. Such a high NAO has been the case during the years 1980 to 2000, the scientists reported.

"We suggest that the large-scale, decadal changes...associated with the NAO are primarily responsible for the ocean heat content changes in the North Atlantic over the past 50 years," the authors concluded.

However, the researchers also noted that this study should not be viewed in isolation. Given reported heat content gains in other oceans basins, and rising air temperatures, the authors surmised that other parts of the world's ocean systems may have taken up the excess heat produced by global warming.

"But in the North Atlantic, any anthropogenic (human-caused) warming would presently be masked by such strong natural variability," they wrote.

Other authors of the report included Richard Williams and Vassil Roussenov of Liverpool University; Susan Leadbetter, previously at Liverpool University but now a postdoctoral researcher with Lozier; Mark Reed, a computational scientist who also works with Lozier at Duke; and Nathan Moore, a former Duke graduate student now at Michigan State University.

Adapted from materials provided by Duke University.