Got a climate question? Ask our expert!
Q. How do we know that the climate is changing? (Laura, London)
A. Although several aspects of climate are changing, temperatures provide the clearest evidence. For many decades, temperature near the surface has been carefully measured at thousands of locations on land and at sea. There are a large number of measurements of temperature close to the Earth's surface which are global in extent, from which we can form a global average, going back to 1860. These all show temperatures higher in the past few years than at any time during the instrumental period, even allowing for measurement uncertainties and gaps in the data.
Global average land and sea temperatures show considerable variability from year to year, but a clear underlying trend which shows rising temperatures until about 1940, a slight downward trend from about 1940–1975, and a rise of about 0.5°C between 1975 and the present day.
Three independent types of temperature measurement — air temperature taken at land climate stations and on ships at night (when the interfering effect of solar radiation is absent) and the temperature of the sea surface — all show good agreement from 1900 until the last couple of decades, when land temperatures have been rising at a faster rate than sea temperatures (as predicted to be the case for a global warming due to increased greenhouse gases).
Temperatures have also been measured in the atmosphere; over the last 50 years or so by weather balloons, and by satellite remote sensing since 1979. In the mid-troposphere, about 5 km above the surface, there has been a global-mean warming. Although data are sparse in tropical regions, according to sensors on weather balloons, there seems to have been little change in temperature in the tropical mid-troposphere over the past 25 years, which is not what models predict. This discrepancy and its implications are the subject of ongoing research.
Q. Isn't another ice age due soon? And won't it counteract global warming? (Tom, Kent)
A. Over the past half million years or more, the world has alternated between ice ages and interglacials (periods between ice ages), with interglacials occurring every 100,000 years or so. We have been in the present interglacial for about 10,000 years. Evidence is strong for this behaviour to be due to changes in the Earth's orbit around the Sun, and the angle of its rotational axis, usually referred to together as 'astronomical forcing of climate'. This theory was formalised by Milankovic in the 1920s, and has been well confirmed by records from ice cores, ocean sediments, etc.
Thanks to our knowledge of orbital mechanics these astronomical changes can be predicted, and it appears that astronomical forcing will be of little significance over the next 40,000 years or so, so the next ice age will be a very long time hence. Thus is it on a very different timescale to man-made global warming and cannot counteract it; if no action is taken to limit fossil-fuel emissions, for instance, climate will have changed substantially by the time the next ice age starts.
Q. I have read numerous articles on Global Warming, deemed by some to be largely 'scaremongering'. I have read only one which questions the fundamental so-called facts about climate change over past centuries: ‘Climate chaos? Don’t believe it!’ by Christopher Monckton, in the Sunday Telegraph.
The argument put forward is that the global warming forecast is fundamentally flawed because the scientific basis for the forecast completely ignores a Medieval Age global warming period, and thus alters all so-called forecasts. The article claims historical climatic data has been manipulated so as to present a totally incorrect picture of the planet's future.
The article says: “So, to the scare. First, the UN implies that carbon dioxide ended the last four ice ages. It displays two 450,000-year graphs: a sawtooth curve of temperature and a sawtooth of airborne CO2 that's scaled to look similar. Usually, similar curves are superimposed for comparison. The UN didn't do that. If it had, the truth would have shown: the changes in temperature preceded the changes in CO2 levels.
“Next, the UN abolished the medieval warm period (the global warming at the end of the First Millennium AD). In 1995, David Deming, a geoscientist at the University of Oklahoma, had written an article reconstructing 150 years of North American temperatures from borehole data. He later wrote: ‘With the publication of the article in Science, I gained significant credibility in the community of scientists working on climate change. They thought I was one of them, someone who would pervert science in the service of social and political causes. One of them let his guard down. A major person working in the area of climate change and global warming sent me an astonishing email that said: We have to get rid of the Medieval Warm Period.'
“So they did. The UN's second assessment report, in 1996, showed a 1,000-year graph demonstrating that temperature in the Middle Ages was warmer than today. But the 2001 report contained a new graph showing no medieval warm period. It wrongly concluded that the 20th century was the warmest for 1,000 years. The graph looked like an ice hockey-stick. The wrongly flat AD1000-AD1900 temperature line was the shaft: the uptick from 1900 to 2000 was the blade.”
Most 'doom and gloom' reports, such as Al Gore's recently released film regarding global warming, provide facts and figures based on the last 50 years or so, which indeed do show a gradual rise in earth's temperatures. This is also true of many other articles concerning global warming, in that only a relatively short period in the earth's history is used for illustration. So to my question: is it all scaremongering? Has scientific data been manipulated in order to give us all a totally incorrect picture of what is to come? I’d be most interested in your opinions. (Peter Curry, Hua Hin, Thailand)
A. No, it is not all scaremongering and the IPCC has not manipulated scientific data in order to give an incorrect picture. You may wish to ask that question about the authors of some newspaper articles, though (How rude! – Ed) or at least question whether out-of-date data is being used.
It is a myth that the IPCC ignores the "Medieval Warm Period" - there is in fact a very comprehensive and balanced discussion in http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch06.pdf, Box 6.4 page 468-469. The possibility of a warmer period between AD1000 and 1200 was first put forward in the 1960s by the eminent climatologist Hubert H Lamb, but the evidence was difficult to interpret and only representative of a few specific locations. A wealth of other studies over the following 40 years reconstructed temperatures over the wider area of the Northern Hemisphere, and these suggest that the period between AD950 and 1100 was very likely to have been warmer than other times in the 12 centuries prior to the 20th Century, but that this "warm period" was still probably cooler than recent decades. See figure 6.10 of the above chapter.
If you want to see data of the last 50 years put into the context of a longer period, please read the above IPCC chapter which discusses climate variations as far back as 400 million years – is that far enough for you ? :-) On the red herring of CO2 rises occurring after temperature rises at times in the past, yes: this is perfectly true, but does not invalidate the mechanism of CO2 rise being a major cause of current climate change (please see my response to David Simpson of Edinburgh).
