Analysis confirms global warming data, accounts for urban heat islands

US scientists have just released evidence of a rise in the average world land temperature, confirming earlier studies and addressing concerns that “heat island” effects as cities developed around long-established temperature recording sites were distorting estimates of global warming.

The research, from the Berkeley Earth Surface Temperature (BEST) project analyses temperature data from 15 sources and was intended to directly address scientific concerns raised by climate sceptics, including the urban heat island effect (UHI), temperature station quality and the risk of data selection bias. The urban heat effect was considered to be at the heart of the allegations of fraud around “Climategate”. The research was released today but has yet to be peer-reviewed.

The Berkeley Earth Surface Temperature (BEST) project was created to make the best possible estimate of global temperature change using over 1.6 billion existing temperature measurements from 39,390 sites.

On an initial analysis of temperatures from randomly-selected stations it found a global warming trend very similar to previous work — by the National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA), and Britain’s Hadley Center Climate Research Unit (HadCRU) — which estimated global warming of about 1.2degC land temperature rise from the early 1900s to the present. The human-caused warming of about 0.7degC since 1957 was similar to the previous work.

It also studied the “quality” recording stations, and found that the warming seen in the “poor” stations was virtually indistinguishable from that seen in the “good” stations. Researchers said this indicated that the systematic biases which had been a cause for most concern could be adequately handled by data analysis techniques, and that world temperature data had sufficient integrity to be used to determine global temperature trends.

Outside groups — some of them describing themselves as climate-change sceptics — had claimed that earlier work had picked records with large temperature increases, perhaps inadvertently by choosing sites with long continuous records. The BEST researchers developed techniques to work with all the available stations, and chose stations at random to eliminate such potential selection bias.

The SMC sought comment from New Zealand climate scientists on the draft paper. Feel free to use these quotes in your stories.

Victoria University of Wellington’s New Zealand Climate Change Research Institute, Professor of Climate Change, Martin Manning comments:

“This paper summarises a new analysis of temperature records from weather stations and their ability to determine long term trends in the global average temperature. It suggests that the potential for temperatures in land based weather stations to have been affected by the development of urban areas could have been overestimated in some previous analyses. So a key finding is that there is no reason to believe that long term global average temperature trends might have been overestimated by others”

“This paper will come under intense scrutiny from some who do not like the results, and had hoped that this new analysis of temperature records might have shown that previous analyses were either wrong or could not be confirmed. Ironically it will also come under scrutiny from many of the climate scientists who have been working in this area for several decades and have been thinking about the quality of temperature records from different perspectives”.

“For example, the paper covers 39,028 different sites for temperature records with 16,132 of these being definitely outside urban areas, but it has not covered the potential implications of changes in the spatial distribution of these sites over time. Similarly there are still some questions about the quality and consistency of data that has been collected from many different types of meteorological organisations and which were not originally set up to determine trends in global average temperature”.

But trends in temperature as seen in meteorological records are only a small part of the evidence for climate change and that was emphasised in the last assessment of the science by the Intergovernmental Panel on Climate Change in 2007. That report showed there were multiple lines of evidence for climate change and to such an extent that, even if this paper by Wickham et al had raised significant questions about meteorological temperature records, there would still have been strong evidence for a major structural change in global climate over the past fifty years”.

“There is clear evidence for accelerating loss of the major ice sheets and glaciers as well as for increases in atmospheric water vapour that are related to more extreme rainfall. The fact that oceans are also warming to depths of 3,000 meters shows that this is not just some redistribution of heat, but can only be explained by a major shift in global energy balance caused by increases in CO2 and other greenhouse gases. It led scientists and governments to agree in 2007 that the evidence for climate change was now “unequivocal”. So the major issue being raised by this paper is whether it is still necessary to scrutinise some of the key aspects of climate change science in this way”.

Senior New Zealand climate scientist, Dr David S Wratt said:

“I would generally not comment on a paper such as this one before it has been through peer review and acceptance by a scientific journal. Nevertheless, this paper does report on a useful piece of work. As the authors state … it supports conclusions from other researchers `that urban warming does not unduly bias estimates of recent global temperature change’. I look forward to reading a final reviewed-journal version”.

NIWA Principal Scientist, Climate Variability and Change, Dr James Renwick, said:

“This paper provides further evidence that urban warming, otherwise known as the ‘urban heat island’ (UHI) effect, has only a negligible influence upon global temperature trends. These findings are consistent with many other studies already published, as discussed by the authors of this paper. Several groups have analysed temperature data from around the globe, independently and with different techniques, and all find the same result: in terms of the global average, urbanisation has little effect upon temperature trends, and that the global average temperature has risen the best part of a degree in the past century. The authors’ selection of “very rural” sites show a warming trend that is consistent with (in fact is slightly larger than) the trend calculated from all sites.

“The authors illustrate nicely that some locations (e.g. metropolitan Tokyo) have warmed very rapidly indeed, but such sites are a small minority. Some other sites show cooling trends – both examples demonstrate that one cannot look at individual locations to determine global trends. It is the global average that counts”.

“It is worth noting, as the authors do in their closing remarks, that this analysis has not used any information on ocean surface temperature. The oceans are of course immune from the UHI effect and yet they show similar upward average temperature trends to those seen over land.

The evidence from the surface temperature record, and from a number of other sources (deep ocean temperatures, sea level, glacier ice, etc) is very clear. The Earth is warming, and the climate is changing”.

Auckland-based international climate scientist, Dr Jim Salinger — who first concluded that the average temperature had warmed significantly during the 20th century after examining temperature data from New Zealand locations over 30 years ago — comments:

“The finding does not surprise me and backs up results from New Zealand non-urban stations. I identified 11 sites … spanning New Zealand where there have been no significant site moves for many decades, also in rural locales. Of these stations, these records go back to at least the 1930s, and for the other two (Raoul Island and Campbell Island) the data go back to 1940 or 1941. The sites are Raoul Island, Tauranga Airport, Ruakura (Hamilton), Gisborne

Airport, Chateau Tongariro, Palmerston North DSIR/AgResearch, Westport Airport, Molesworth, Queenstown, Invercargill Airport and Campbell Island”.

“The trend over the 78 year period 1931-2008 is close to 1degC. Warming is especially strong at sites such as Chateau Tongariro, Molesworth and the Hermitage at Mt Cook, sites that can have no possibility of ‘urban heat islands’. So these temperature increases are very similar, if not stronger than the NZ 7-station series based on Auckland, Masterton, Wellington, Hokitika Airport, Nelson Airport, Lincoln and Dunedin”.

Comments gathered by our colleagues at the Australian Science Media Centre:

Prof Dave Griggs is CEO of ClimateWorks Australia & Director of the Monash Sustainability Institute, Melbourne, comments:

“The paper confirms previous work that the observed warming of the Earth since the beginning of the 20th Century cannot be attributed to the urban heat island (UHI) effect. This comes as no surprise to climate scientists who take great care to take account of this effect in their work. It also confirms findings from other studies using very different methods. For example, looking at temperature trends on windy days and calm days also show no difference contrary to what you would expect from the urban heat island as the heat would build up more on calm days. So, hopefully this paper will help to put this urban (heat island) myth to bed.”

Prof Neville Nicholls is ARC Professorial Fellow in the School of Geography and Environmental Science at Monash University, Melbourne, comments:

“My first reaction on hearing the result of this study, namely that urbanization has NOT led to exaggerated estimates of global warming, was certainly not one of surprise! Those of us who have spent years checking the temperature data and using these to estimate global warming have always checked to ensure that this was not an issue. And even if you were worried about this possible issue, the fact that temperatures measured on small islands were showing similar warming, as do ocean temperatures and alpine snow and glacier shrinkage, and sea-ice retreat, should have put your mind at rest. Even satellite measurements of global temperature show similar warming to the land-based thermometers.

“Only those most desperate to dismiss global warming have tried to blame urbanization for the observed warming. But I guess it is always good to have yet another group confirm decades of work by climate scientists. Perhaps this will finally put to rest the furphy that the warming is caused by urbanization. I won’t hold my breath though. I guess those spreading misinformation about warming will just move on to another one of their furphies.”

Comments gathered by our colleagues at the UK Science Media Centre:

University of Exeter, Professor of Climate System Dynamics, Prof Peter Cox, comments:

“These studies seem to confirm the global warming estimated from the existing datasets, which is pleasing but not exactly a surprize to those of us who know how carefully the existing datasets are put together. It is surprising however that the authors believe that this news is so significant that they can’t wait for peer-review, especially when their conclusions aren’t exactly revolutionary.”

Department for Environment, Food and Rural Affairs (Defra) Chief Scientific Adviser Prof Robert Watson, comments:

“While I have not seen the paper, and recognise the results have not been peer reviewed, I am very encouraged that their preliminary analysis supports previous analyses of the UK Hadley Center-UEA Climatic Research Unit, and the US analyses of NASA Goddard Institute of Space Sciences, and NOAA. All previous analyses paid particular attention to removing the urban heat Island effect – this new work appears to show that they did this successfully”

University of East Anglia Climatic Research Unit Director Prof Phil Jones, comments:

“I look forward to reading the finalised paper once it has been reviewed and published. These initial findings are very encouraging and echo our own results and our conclusion that the impact of urban heat islands on the overall global temperature is minimal.

University College London (UCL) Climate Science Professor Chris Rapley comments:

“I think it would be unwise to comment on these specific results until the peer review process has been completed. If we have learned anything over the last couple of years it is surely that ensuring the rigour of the science is paramount. Having said that, any new and independent evidence vindicating the conclusions of scientists working in this field could only be helpful”

Grantham Institute for Climate Change and the Environment (LSE), Policy and Communications Director, Bob Ward, comments:

“This new study confirms what we already knew. The warming of the land areas of the Earth since the 19th century cannot be explained by the impact of cities growing to engulf rural weather stations. So-called ‘sceptics’ should now drop their thoroughly discredited claims that the increase in global average temperature could be attributed to the impact of growing cities, which create an urban heat island effect. This claim was always dubious as oceans also show the same level of warming as land areas. The warming of the Earth is unequivocal and, as every major scientific organisation in the world has pointed out, the overwhelming evidence indicates that this warming is being driven by the unarguable increase in greenhouse gas concentrations in the atmosphere. More broadly this study also proves once again how false it was for ‘sceptics’ to allege that the e-mails hacked from the University of East Anglia proved that the Climatic Research Unit’s land temperature record had been doctored. Several independent inquiries, and now this study, have shown that allegation to be entirely untrue. It is now time for an apology from all those, including US Presidential hopeful Rick Perry, who have made false claims that the evidence for global warming has been faked by climate scientists.”

Grantham Institute for Climate Change and the Environment (LSE), Director, Professor Sir Brian Hoskins, comments:

“If and when such a study is peer-reviewed and published, hopefully we can then focus on the implications for the future of this warming rather than wrangling over whether the warming is really there.”