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Monday 15 June 2009 (updated 11am AEST, Tuesday 16 June)

RAPID ROUNDUP: Link between CO2 and temperature - experts respond

As the Carbon Pollution Reduction Scheme is debated in Parliament this week, climate scientists attempt to succinctly answer Senator Stephen Fielding’s question: why have carbon dioxide concentrations over the last decade been going up but temperatures have not?

Feel free to use these quotes in your stories. Any further comments will be posted here. If you would like to speak to an expert, please don’t hesitate to contact us on (08) 8207 7415 or by email.

Kevin Hennessy is a Principal Research Scientist with the CSIRO Climate Adaptation Flagship.

"NASA, NOAA and the Hadley Centre have all produced global surface temperature records. They all show global temperatures fluctuate from year to year and decade to decade. They also show a warming trend from 1910-1940, followed by a slight cooling from 1940-1950, then another warming trend from 1950-2008. Two of the three temperature records have 2005 as the warmest year, while the third has 1998 slightly warmer than 2005. The high temperatures in 1998 were partly caused by a strong El Niño event that year.

Data from 1998-2008 tell us nothing about long term trends; the period is simply too short. In a peer-reviewed scientific paper, Easterling and Wehner (2009) http://www.agu.org/pubs/crossref/2009/2009GL037810.shtml state:

'Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming'.

The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) concluded that warming of the climate system is unequivocal. The last decade has been the warmest on record. The surface of the Earth has now warmed by more than 0.7˚C since the start of the twentieth century, and the warming is likely to continue through the 21st century, with associated changes in extreme weather events. After considering all evidence available to 2007, the IPCC concluded that ‘average Northern Hemisphere temperatures during the second half of the 20th century are very likely higher than during any other 50 year period in the last 500 years and likely the highest in the at least the past 1300 years’. This includes the Medieval Warm Period around 1100 AD.

The IPCC has assessed the contributions of greenhouse gases, aerosols, ozone depletion, land-use change and solar variations to global warming experienced since 1750. The greenhouse gases are clearly dominant, with an uncertain cooling contribution from aerosols, and a warming component from solar variability that is just 5% of the size of that from greenhouse gases.

Climate models driven by natural factors such as variations in solar irradiance and volcanic eruptions do not simulate the global warming that we have experienced since 1950. Add greenhouse gases and aerosols to the simulations and they match observations remarkably well. The peer-reviewed literature assessed by the IPCC supports the conclusion that it is very likely that most of the warming observed since the mid-20th century is due to anthropogenic increases in greenhouse gases. For more information, see IPCC (2007, Working Group 1 chapter 9)."

Professor Neville Nicholls is Professorial Fellow in the School of Geography and Environmental Science at Monash University, Victoria and has published more than 150 refereed journal papers and book chapters on the nature, causes, impacts and predictability of climate variations and change.

1. Global temperatures HAVE increased over the past decade (1999 to 2008). Global average temperature over land has increased about 0.3C over this decade (linear trend over the decade). Global average ocean temperature has increased about 0.1C over the decade, as has the satellite-measured lower tropospheric temperature.

2. The warming has been dampened by the occurrence of a La Niña in 2007/08, ie the last couple of years of the decade. La Niña events are usually followed by cool global temperatures. If the temperature trends were calculated without these last two years the ocean and lower atmosphere warming trends would have been 0.2-0.3C per decade.

3. The strong effect of El Niño and La Niña on global temperatures means it is unlikely that trends over periods as short as a decade or so would closely match the warming expected from gradual increases in the atmospheric concentration of greenhouse gases.

4. Nevertheless, the warming trends over the last decade are about what might have been expected, given the approximately 5% increase in the CO2 concentrations in the atmosphere over the decade. The IPCC AR4 projections of global temperature increases over the two decades starting in 2000 were for about 0.2C warming per decade.

5. Every year this century (2001-2008) has been amongst the 10 warmest years on record since instrumental records began in the middle of the 19th century.

Professor Matthew England is an ARC Federation Fellow and Joint Director of the Climate Change Research Centre (CCRC) in the Faculty of Science at The University of New South Wales

“It is completely consistent with our understanding of the climate system that we will experience a decade or more of minimal warming – or even slight cooling – in the background of a long-term accelerating warming trend. We saw this in the 20th Century and it will be a feature of the coming century even though global air temperatures will warm at an accelerated rate over the long-term.

This is all about the capacity of the earth’s climate to vary over interannual to decadal time-scales. In this case, a very strong El Niño in 1998 made that year particularly warm, and since that time there have been a few La Niña years and the El Niños that have occurred have been relatively weak.

People are cherry picking the post 1998 period to conveniently suggest that global warming has stopped. This isn’t the case at all – the decade that immediately followed 1998 was the warmest decade ever recorded. Scientists from the US recently showed that even under a century-long warming trend of around 4 degrees C – that is, toward the very high end of the IPCC range – we will still see the record punctuated by select ten year periods of no trend or slight cooling.”

Professor Roger Stone is Professor in Climatology and Water Resources and Director of the Australian Centre for Sustainable Catchments at the University of Southern Queensland, Toowoomba.

“This is due to co-existing Natural variability due to a few more La Niña events than normal during this period (although even these are warmer than they used to be!). This is what would be expected.

Given a likely El Niño now developing for the next 12 months it will be interesting to see how this will shift the 'debate' (heaven help many areas of southern and eastern Australia this coming 12 months - but that is another story).

The US NCAR trend data clearly show a long-term warming trend globally over the past 100 years but with some year to year variability that one would expect with La Niña and El Niño events. So, if you take a few years or even a decade for analysis then you may not see long term trends, especially if you don't want to see them.”

Associate Professor Kevin Walsh is in the School of Earth Sciences at Melbourne University. His research interests include climate variability and tropical weather systems. He is also past president of the Australian Meteorological and Oceanographic Society.

“The global average temperature is quite closely related not only to carbon dioxide amounts, but also to the current state of ENSO (La Niña /El Niño). If you look at temperatures over the past decade, 1998 was a big El Niño year and El Niños make the global average temperature go up. So the temperature trend over the last decade is not big. We expect the global average temperature in any one year to be up and down a bit from the long-term trend because of ENSO. The long-term trend over the last few decades, though, is very clearly upwards and we expect it to continue upwards in the near future.”


Professor Andy Pitman is Joint Director of the Climate Change Research Centre (CCRC) in the Faculty of Science at The University of New South Wales.

“We have been in a long La Niña. This cools the globe several tenths of a degree. Climate science has never hinted that there is a given amount of warming per unit of CO2 on a year-to-year basis. Natural variability imposes additional warming and cooling on this long term trend. La Niña cools and temporarily hides the warming caused by CO2.”

Professor Peter Schwerdtfeger is Emeritus Professor of Meteorology at Flinders University. Please note pictures referred to below are available on request from the AusSMC.

“One of the problems with large social, including scientific, groups is that individual thinking tends to become submerged by the group psyche.

While CO2 may be playing a significant part in maintaining the Earth’s heat balance because it absorbs and radiates in the infra-red spectrum, I and others are convinced that particulate pollution, which is a by-product of burning fossil fuels, plays an even greater role in the atmosphere.

The two attached images respectively show typical power stations (Churchill in Gippsland Vic.) and an analysis of satellite data over SE Australia by Rosenfeld and Lensky together with my comments. Professor Daniel Rosenfeld of the Hebrew University in Jerusalem has been awarded a medal for his work by the American Meteorological Society but not accorded sufficient attention in Australia. While Rosenfeld has been principally concerned with recent reductions in Australian rainfall, the Earth’s temperature is also strongly dependent on rainfall and precipitation processes.

Altogether, I believe that it is important for scientists to remain open to broader views and avoid the intellectual danger of being locked into tunnel-vision through “group obsessions”.”

Dr Paul Fraser is a Chief Research Scientist at The Centre for Australian Weather and Climate Research – A partnership between CSIRO and the Bureau of Meteorology.

“Global data from the UK Meteorological Office show that 2008 was the fourth-coolest year over the past 12 years, but warmer than any year in the 1980s. In fact, over the past 200 years of global instrumental records, 2008 was the ninth hottest year and likely the ninth hottest over the past several thousand years. The exact sequence of ‘hottest’ years is not important – however what is alarming is that the 12-year period 1997-2008 contains the 11 hottest years ever recorded, 1995 and 2000 being the twelfth and thirteenth hottest years ever. The 11-year (1998-2008) global average temperature is the warmest 11-year period average temperature ever recorded.

The long-term warming trend over the past 100 years is obvious. Global temperatures have cooled since 2005 – a four year cooling trend – but the records show that there have been six such four-year-or-more cooling sequences and seven four-year-or-more warming sequences since 1900.

The science of global climate change predicts that we should expect a long-term trend in global warming, driven by accumulating greenhouse gases and changes in aerosols, modulated by cyclical influences such as El Niño/La Niña (3-5 years) and the solar cycle (11 years), as well as unpredictable events such as volcanic eruptions.

The fact that two or three summers are particularly cool (or warm) cannot be used to either prove or disprove the science of global warming. A much longer-term view is required.

I am not aware of any peer-reviewed paper in a respectable geophysical journal (sanctioned by the American Meteorological Society or the Royal Meteorological Society for example) that argues a convincing explanation for temperature trends over the past 200 years that does not involve the trends in greenhouse gases and aerosols driven by fossil fuel consumption and inter-annual variations in greenhouse gases and aerosol loading, responding to largely El Niño /La Niña climate cycles, with a small input from an increasing solar output (perhaps responsible for about 5% of the temperature trend). There are many papers supporting this explanation for the observed temperature trends.”