Cooling waters in the tropical Pacific Ocean appear to be a major factor in dampening global warming in recent years, scientists said on Wednesday.
Their work is a big step forward in helping to solve the greatest puzzle of current climate change research – why global average surface temperatures, while still on an upward trend, have risen more slowly in the past ten to fifteen years than previously.
Waters in the eastern tropical regions of the Pacific have been notably cooler in recent years, owing to the effects of one of the world’s biggest ocean circulatory systems, the Pacific decadal oscillation.
Many people are aware of the El Niño and La Niña weather systems, which affect the Pacific and bring hotter and stormier or cooler weather in cycles of just a few years, and can have a strong effect on global weather. But few are aware that both of these systems are just part of the much bigger Pacific decadal oscillation, which brings warmer and cooler weather over decades.
The system is now in a cooling phase, scientists have noted, which could last for years. The last such phase was from the 1940s to the 1970s.
The new study by the Scripps Institution of Oceanography and supported by the US government’s National Oceans and Atmospheric Administration (NOAA), published in the journal Nature, has linked the “pause” in global warming with the Pacific oscillation.
Dan Barrie, programme manager at NOAA, called the research “compelling” and said: “[It] provides a powerful illustration of how the remote eastern tropical Pacific guides the behaviour of the global ocean-atmosphere system, in this case exhibiting a discernible influence on the recent hiatus in global warming.”
In winter, the effect of the cooler phase of the oscillation on the northern hemisphere is to depress temperatures slightly; but in summer, the cooler waters in the equatorial Pacific have less impact on the northern hemisphere’s weather.
The scientists, using computer models, compared their results with observations and concluded that global average annual temperatures have been lower than they would otherwise have been because of the oscillation. But the observed higher summer temperatures of recent years show more of the true effects of global warming, according to the research. Global average temperatures are taken over the whole year, obscuring the effect of this seasonal variation.
Shang-Ping Xie, professor of environmental science at Scripps, said: “In summer, the equatorial Pacific’s grip on the northern hemisphere loosens, and the increased greenhouse gases continue to warm temperatures, causing record heat waves and unprecedented Arctic sea ice retreat.”
Dr Alex Sen Gupta, of the Climate Change Research Centre at the University of New South Wales, who was not part of the study team, said: “The authors have set up some elegant experiments using a climate model to test whether a natural oscillation that has gone through a large swing in the tropical Pacific Ocean over the last decade can explain the recent halt in surface global warming … the new simulation accurately reproduces the timing and pattern of changes that have occurred over the last four decades with remarkable skill. This clearly shows that the recent slowdown is a consequence of a natural oscillation.”
The role of oceans in regulating the planet’s temperatures has taken on a greater significance in climate change research, as not enough is yet known about how ocean currents and the circulation of warmer surface water to the deep oceans below affect the weather and climate.
Research indicates that oceans have absorbed much of the heat and about a third of the additional carbon dioxide pumped into the air from pre-industrial times. This has an effect – the thermal expansion of the oceans is likely to be the biggest factor behind sea level rise, and the absorption of carbon dioxide is making the oceans more acidic.
Scientists also think that the circulation of heat from the top layers of the ocean, which have been most affected to date, to the deeper oceans below may be another factor behind the “hiatus” in global warming. What the full effects of this exchange of energy may be, particularly on ocean currents, is not yet known.
Researchers have called for more observations of the ocean, including many more buoys and underwater readings.
The slowdown in the upward march of global average temperatures has been greeted by climate sceptics as evidence that the climate is less affected by greenhouse gases than thought. But climate scientists are much more cautious, pointing out that the trend is still upwards, and that the current temperature rises are well within the expected range. Past temperature records and computer predictions both show that periods of slower rises are to be expected as part of the natural variability of the planet’s climate. gns