Rising temperatures are linked to a decrease in carbon dioxide (CO2) absorption by tropical forests, according to a 50-year study published today.
Greenhouse gases, such as CO2, contribute to global warming, sea level rises and extreme weather events, previous studies have shown.
Forests absorb CO2 during photosynthesis and release it during respiration.
The new NASA-lead study, conducted with the CSIRO and published in the journal Proceedings of the National Academy of Sciences, builds on a growing body of evidence that suggests global warming will accelerate as time goes by.
The researchers analysed data on global atmospheric carbon dioxide concentration and climate variability between 1959 and 2011, which included El Niño years characterised by higher temperatures and lower average rainfalls.
The researchers found that a tropical land surface temperature rise of one degree Celsius led to an average extra 3.5 Petagrams of CO2 being pushed into the atmosphere per year. A Petagram is a billion tonnes.
"Tropical forests are carbon sinks but when it gets hotter, they become less efficient in absorbing CO2 from the atmosphere. We are learning very clearly that tropical forests do not like to be any hotter than they are. As soon as you increase the temperature, they perform less well as carbon sinks," said study co-author Pep Canadell, Executive Officer of the Global Carbon Project.
"Many processes involved in this response are the same as what is known as the carbon-climate feedback, which it is thought will lead to an acceleration of carbon emissions from vegetation and soils and into the atmosphere under future climate change."
Exacerbating the problem
Steve Sherwood, Director of the Climate Change Research Centre at the University of New South Wales, said the findings were important.
"The signals they see are a good way to test climate models that include an interactive carbon cycle. Also, their findings help explain the well-known, natural, year-to-year variations in the rate of carbon dioxide buildup, and in so doing, they increase the likelihood that global warming in the future will cause more of the carbon dioxide now stored in tropical plants and soils to go into the atmosphere," said Professor Sherwood, who was not involved in the study.
"This will exacerbate our own emissions of this gas, and therefore will make human-caused climate change and ocean acidification a bit worse than these problems would otherwise have been. This is not a new conclusion, but their results increase our confidence in it."
Sea levels could rise by 2.3 metres for each degree celsius that global temperatures increase and they will remain high for centuries to come, according to a new study by the leading climate research institute.
Anders Levermann said his study for the Potsdam Institute for Climate Impact Research (in press) was the first to examine evidence from climate history and combine it with computer simulations of contributing factors to long-term sea-level increases: thermal expansion of oceans, the melting of mountain glaciers and the melting of the Greenland and Antarctic ice sheets.
Scientists say global warming is responsible for the melting ice. A U.N. panel of scientists, the IPCC, says heat-trapping gases from burning fossil fuels are nudging up temperatures.
"We're confident that our estimate is robust because of the combination of physics and data that we used," Levermann told Reuters. "We think we've set a benchmark for how much sea levels will rise along with temperature increases."
Sea levels rose by 17 cm last century and the rate has accelerated to more than 3 mm a year, according to the IPCC. A third of the current rise is from Antarctica and Greenland.
Almost 200 governments have agreed to limit global warming to less than 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times and plan to agree, by the end of 2015, a deal to curb emissions.
Global average surface temperatures have risen by 0.8 degrees since the Industrial Revolution and the IPCC has said temperatures are likely to be 0.4 to 1.0 degrees Celsius warmer from 2016-35 than in the two decades to 2005.
"In the past there was some uncertainty and people haven't known by how much," Levermann said. "We're saying now, taking everything we know, that we've got a robust estimate of 2.3 meters of rising sea per degree (Celsius) of warming."
Some scientific studies have projected sea level rise of up to 2 metres by 2100, a figure that would swamp large tracts of land from Bangladesh to Florida.
David Vaughan, head of the Ice2sea project to narrow down uncertainties about how melting ice will swell the oceans, has said sea levels would rise by between 16.5 and 69 cm under a scenario of moderate global warming this century.
Vaughan told Reuters the biggest impact rising seas will have is that storms will be more destructive in the near future.
"It's not about chasing people up the beach or the changing shape of coastlines," he said. "The big issue is how the storms will damage our coasts and how often they occur. That'll increase even with small levels of sea rise in coming decades."
"Continuous sea-level rise is something we cannot avoid unless global temperatures go down again," Levermann said. "Our results indicate that major adaptation at our coastlines will be necessary. It's likely that some currently populated regions can't be protected in the long run."
Power plants across the US are at increased risk of temporary shutdown and reduced power generation as temperatures and sea levels continue to rise and water becomes less available, the country's Department of Energy said.
"As President Obama said in his speech last month, climate change is happening," said spokeswoman April Saylor in a statement. "As climate change makes the weather more extreme, we have a moral obligation to prepare the country for its effects."
The report calls on federal, state and local governments to more urgently prepare critical infrastructure - particularly coal, natural gas and nuclear plants - for the compounded risks posed by floods, storms, wildfires and droughts.
"All of our science goes in one direction: the damages are going to get worse," said DOE assistant secretary Jonathan Pershing. "It will take dozens of actors from government and private sectors planning what to do and how to make it cost effective."
The report notes that average temperatures have increased 1.5 degrees fahrenheit since 1900. More than 130 extreme weather events costing $US1 billion or more in damage have occurred since 1980.
It says that 2012 was the second most expensive year for weather and climate disasters in the US, with $US115 billion in damage from Hurricane Sandy and extended drought.
Higher peak electricity, costing consumers $US45 billion, will require an additional 34 gigawatts of new power generation capacity in the western United States by 2050. And as infrastructure ages, storm-related power outages are likely to become increasingly frequent, at an annual cost of $US20 billion to $US50 billion.
"More and more communities are analysing vulnerabilities and their risks, and developing plans in response to those risks," said Brian Holland of the International Council for Local Environmental Initiatives. Holland said more state and federal support is needed for communities, who also pursue private funding.
Greenpeace USA spokesman Robert Gardner said the administration's focus should primarily be transitioning to wind and solar technology, not relying on fossil fuels and nuclear energy.
"The question is why the Department of Energy is really focusing on continuing the problem which has caused this tidal wave of global warming," said Gardner.
Benjamin Cole of the American Energy Alliance, which lobbies for oil and natural gas, said that climate predictions should not be used to justify the "sweeping changes" of Obama's energy proposals, and that alternative energy has not yet lived up to promises.
"We can't continue to dole out money we don't have," for alternative energy, he said.
The carbon tax and a continued slump in electricity demand have cut carbon dioxide emissions of eastern Australia's power sector by 12.2 million tonnes in the past year, new research shows.
The carbon emission index produced by consultants Pitt & Sherry shows emissions from national electricity generators serving the ACT, NSW, Tasmania, Victoria and parts of Queensland and South Australia, fell 6.9 per cent for the year. The drop outpaced a 2.2 per cent slide in overall power demand.
The carbon tax, which rose from $23 a tonne to $24.15 on July 1, had its biggest impact in promoting hydro power, which grabbed the most market share from coal-fired power producers.
Adjusting for power consumed by the coal-fired plants, their share of total supply to the market dropped from 75.1 per cent in the year to June 30, 2012 to 72.5 per cent last financial year.
''That's the lowest for coal for many, many decades,'' said Hugh Saddler, principal consultant at Pitt & Sherry.
While hydro's share jumped to 9.6 per cent from 7.6 per cent, supply from wind farms rose to 5.6 per cent from 5.5 per cent. Gas also gained, rising to a share of 12.2 per cent from 11.8 per cent in the previous year, Pitt & Sherry said.
Determining the contribution of the carbon tax to cutting emissions and curbing electricity use is difficult, not least because other factors are involved, Dr Saddler said.
These include energy efficiency programs and rapid spread of solar photovoltaic panels, while the closure of the Kurri Kurri aluminium smelter, in NSW, and other manufacturing cutbacks also sapped demand.
And the fraught political debate about carbon price has probably made people think more about power use.
''People are becoming aware that it's quite easy to reduce their consumption to some extent,'' Dr Saddler said.
The planet has warmed faster since the turn of the century than ever recorded, almost doubling the pace of sea-level increase and causing a 20-fold jump in heat-related deaths, the United Nations said.
The decade through 2010 was the warmest for both hemispheres and for land and sea, the UN's World Meteorological Organisation said Wednesday in an e-mailed report (pdf here) examining climate trends for the beginning of the millennium. Almost 94 per cent of countries logged their warmest 10 years on record, it said.
"The decadal rate of increase between 1991-2000 and 2001-2010 was unprecedented," WMO Secretary-General Michel Jarraud said in a statement. "Rising concentrations of heat-trapping greenhouse gases are changing our climate, with far-reaching implications for our environment and our oceans."
The report underlines the challenge the globe faces in containing temperature gains since industrialisation to the 2-degree Celsius (3.6 Fahrenheit) ceiling set by UN climate-treaty negotiators. The planet is on course to warm by 4 degrees by 2100 because emissions are still rising, the World Bank says.
Deaths from heatwaves surged to 136,000 in the 10-year period from fewer than 6,000 the previous decade, mainly a result of extreme temperatures in Europe in 2003 and in Russia in 2010, according to the WMO. A total of 511 disasters related to tropical cyclones killed 170,000 people and caused $US380 billion ($419 billion) of economic damage. Deaths from storms and floods fell.
"Given that climate change is expected to lead to more frequent and intense heatwaves, we need to be prepared," Jarraud said. "Despite the significant decrease in casualties due to severe storms and flooding, the WMO report highlighted an alarming impact on health and mortality rates caused by the European and Russian heatwaves."
The average global temperature for 2001-2010 was 14.47 degrees Celsius, according to the report. That's 0.21 degree warmer than 1991-2000 and 0.79 degree warmer than 1881-1890. The increase was recorded even without any "major El Nino" event during the decade, the WMO said. El Nino is a periodic warming of waters in the Pacific that pushes up global temperatures.
Sea levels rose at 3 millimeters (0.12 inch) a year, almost double the 20th-century rate of 1.6 millimeters a year.
Seas rise as warmer temperatures cause the water to expand and ice sheets in Greenland and Antarctica and alpine glaciers around the world melt. Record sea-ice melt in the Arctic Ocean doesn't raise seas because the ice already rests on the ocean.
Australia remains on course to post one of its hottest years on record even as conditions favour above-average rainfall across much of the country, the Bureau of Meteorology said.
In the first six months of 2013, the country's average daily maximum temperature was 1.05 degrees higher than the long-term norm, placing the period behind only 2005 in terms of unusually warm conditions.
While above-average rainfall across much of south-eastern Australia during June kept a lid on maximum daily temperatures for that period, the increased cloud cover meant minimum temperatures were above normal in all states and territories for the month.
High pressure systems - with the warmer conditions they typically bring - have continued to dominate weather patterns over much of central and southern Australia, said Rob Smalley, a climatologist at the weather bureau's National Climate Centre.
Air pressure has "been higher than normal, and that would probably [see] less of the frontal activity bringing the cooler conditions through," Dr Smalley said.
Neutral conditions in the key El Nino Southern Oscillation weather system over the Pacific Ocean suggest that calendar year 2013 is likely to be among Australia's hottest in records going back more than 100 years.
"If Australia were to maintain an anomaly of at least one degree, or a little higher, we'd actually approach or come close to exceeding the previous mean temperature record," Dr Smalley said.
Australia has experienced a series of relative heatwaves moving over much of the continent for the past nine months or so.
As a result, Australia's average maximum temperature for the 12 months to June 30 was the hottest on record, exceeding the previous high set in 2002-03.
The anomaly for 2012-13 is 1.18 degrees compared with the 1.1-degree anomaly for 2002-03, Dr Smalley said.
The bureau's national temperature outlook suggests cooler-than-normal days for the coming three months, with Tasmania and the south-western tip of WA two of the exceptions. Night-time temperatures, though, should continue to track well-above average for much of the country, including most of Victoria, south-eastern NSW and all of Tasmania.