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Data Underestimates Global Warming Says Research

14.11.2013
14.11.2013 09:25 Age: 4 yrs
By: Leon Clifford

Missing data from polar regions means that one of the key datasets used by climate scientists may be underestimating the trend in global warming by more than a half, says a new paper published in the Quarterly Journal of the Royal Meteorological Society.

1. Temperature data from the Met Office (thin lines) compared to the optimal Cowtan and Way (2013) global reconstruction (thick lines). The straight red lines indicate the trend over the past 16 years in the respective data. The background image illustrates the coverage of the Met Office data, with colours indicating geographical temperature trends. The Arctic is warming much faster than the rest of the planet. Click to expand.

2. Comparison of the hybrid global temperatures to the Met Office values, showing the Met Office uncertainty bounds.Click to expand.

3. Comparison of the Met Office 16 year trend maps to 5 other sources: The NASA infilled data, the UAH satellite data and three weather models, and to the new hybrid reconstruction. Click to expand.

4. If the Met Office sea surface temperature corrections are applied to the NASA data, the resulting 16 year trend (i.e. 1997-2012) is 0.103°C/decade. Using the Met Office data and a similar reconstruction method the authors obtain a similar trend of 0.108°C/decade. The best reconstruction including the satellite data shows a trend of 0.119°C/decade. Click to expand.

5. Probability distribution of trend values, showing the probabilities that global warming has stopped or accelerated. Click to expand.

Gaps

The problem identified by Cowtan and Way is caused by the method of analysis used in the HadCRUT data. Essentially, the HadCRUT analysis ignores those areas of the globe where it does not have data, which are particularly common in the Arctic where there has been faster warming than elsewhere on the globe. Filling in the gaps in the missing data using a clever algorithm indicates a warming trend of 0.12oC per decade since 1997 rather than the 0.05o0C per decade since 1978 although satellite temperature monitoring has its own problems in polar regions. But it would be higher than the 0.08o

Implication

Here is what Kevin Cowtan says on his own website:

 

The Met Office temperature record, HadCRUT4, is widely quoted as a measure of global warming. However observations are only available for about 84% (five sixths) of the planet. The omitted region includes the Arctic, which is warming much faster than the rest of the planet. As a result, HadCRUT4 underestimates the rate of warming in recent years. 

We have developed a method for using satellite data to fill in the gaps in the Met Office data. Our global record suggests that surface temperatures have been warming two and a half times faster than Met Office estimates over the past 16 years. Temperature trends starting in 1997 or 1998 are particularly affected.

The temperature change for any individual year is not very large (and less than the Met Office uncertainty estimates), but together they make a significant difference to recent temperature trends. This highlights the danger of drawing conclusions from trends calculated over short periods.

Scientific context

Climate scientists have traditionally looked at climate over long periods - 30 years or more. However media and public interest in shorter term trends has focussed attention on the past 15-16 years. Short term trends are much more complex because they can be affected by many factors which cancel out over longer periods. To interpret the 16 year trend, it is necessary to take into account all of these factors, including volcanoes, the solar cycle, particulate emissions from the far East and changes in ocean circulation. The bias addressed by this paper is just one piece in that puzzle, although a largish one.

Most of the other factors affecting the recent temperature trend were discussed in a recent Met Office meeting.

Previous research

Evidence for underestimation of recent temperature trends

The Met Office temperature data only covers about ⅚ of the planet, with the largest unobserved regions being Antarctica, the Arctic, and some continental interiors. The Arctic is particularly important since it is warming much faster than the rest of the planet. There are a number of sources of evidence for the rapid warming of the Arctic:

  • A few high latitude weather stations.

  • Satellite observations of temperatures in the lower atmosphere.

  • Reanalyses, created by running modern weather models on historical data to infer the state of the atmosphere.

Other supporting evidence not used in this work include:

  • Satellite radiometer data (for example Hansen 2006, Chapman et al 2013).

  • Weather balloons (Thorne et al 2005).

  • Climate models (see for example a comparison by Dr Ed Hawkins).

  • Arctic ice loss (although this is strongly influenced by other factors).


How do the results compare to other measures of global temperature?

The other widely quoted measures of global mean surface temperature are the GISTEMP record from the National Aeronautics and Space Administration (NASA) and the NCDC record from the National Oceanic and Atmospheric Administration (NOAA). Our results show slightly faster warming over the past 16 years than the NASA data, and significantly faster warming that NOAA. What is the reason for this discrepancy?

There are two known observational biases which impact recent temperature trends. Coverage bias, addressed in this paper, impacts the Met Office and NOAA data. A bias in sea surface temperature observations (arising from a recent transition from ships to buoys) is present in the NASA and NOAA data. We anticipate that when both of these biases are corrected, the resulting records will show even better agreement.

Did the Met Office get it wrong?

No. The Met Office have been reporting the existence of this bias since 2009, although the issue has not received widespread media coverage. The Met Office also provide uncertainty estimates for their temperature data: our results fall within the 95% confidence intervals of the annual data.

We started from a desire to understand the differences between different versions of the surface temperature record. As a result it was probably easier for us to tackle the problem as outsiders than for someone in one of the existing groups.

We based our final reconstruction on the Met Office data because they are the only group to have addressed the (much harder) problem of sea surface temperature bias.

What about the IPCC?

The IPCC does not perform original research; it can only summarise research from the peer-reviewed literature. The deadline for submission of papers to be considered for inclusion in the IPCC AR5 report was July 2012. Much of the work on recent temperature trends - including our paper - postdates this deadline.

Is the 16 year trend statistically significant?

About the authors

Kevin Cowtan trained in theoretical physics at the University of York. Apart from sabbaticals to San Diego Supercomputer Center he has spent most of his career working in the Chemistry Department at York. He develops computational methods for X-ray crystallography, with a particular focus on image reconstruction and feature recognition. His work is highly cited in the chemistry and biology literature, and his online teaching tools are also very widely used within the field and beyond. He took on this project in his spare time because the problem has synergies with his previous work and is of significant public interest.

Robert Way is a PhD student at the University of Ottawa. He holds a Master of Science from Memorial University of Newfoundland where he studied the climatic sensitivity of Torngat Mountain Glaciers. His current project is aimed at spatially modeling the distribution of permafrost in the eastern Canadian sub-Arctic. He is also a contributor to the IRIS4 working group on climate variability and change in the eastern Canadian subarctic. As an Inuit descendant from the Canadian sub-Arctic his field experience and knowledge of the cryosphere provided an important complement to Dr Cowtan's computational and analysis skills.

"No difficult scientific problem is ever solved in a single paper. I don't expect our work to be the last word on this, but I hope we have advanced the discussion." - Dr Cowtan


Abstract from the paper:

Citation:

here.

Source University of York website here.