About this page
Climate change is a subject that generates a great deal of heat and emotion often conflated with politics. Three positions seem to be emerging in the general perception if not within the climate science community.
This is a re-write of a page written, originally, in 2004. Hopefully it will be briefer and more germane than the original.
Climate is changing. Given the potential dangers and the perceived costs of taking or not taking action, it is not surprising that there are widely disparate views among the general, non-specialist public. At one extreme we have the doom-mongers who predict the end of the world as we know it. At the other extreme are the deniers who seem to be incapable of accepting the reality of climate change let alone why it is changing. Then we have those that accept what has happened as fact but are reluctant to go further an agree that there may be real dangers. In effect, they are closet deniers.
Talking to climate scientists and reading peer reviewed papers, it is clear that none of these positions is correct or sensible.
Is climate changing? The answer must always be "Yes". Climate has always been changing. But, the impotent questions are:
- Are the current changes really unusual?"
- Why is it changing?
- Is man responsible?"
- And, if so can any action be taken?
Why climate changes
On my Numerical Weather Prediction page I describe briefly how the atmosphere works. As will be seen from that and this diagram it is a complex system driven by heat from the sun as the only external source of energy to the climate system.
(From IPCC AR5, Woking Group 1 Report, Chapter09_FINAL.pdf Chapter 9, 1913.)
Two factors determine the heat input to the atmosphere. The first is how much heat reaches the top of the atmosphere. The second is how that heat is converted into air movement that we experience as the weather and climate system.
How much heat reaches the top of the atmosphere?
This depends on two factors, the distance of the earth from the sun and how much heat the sun is emitting.
The first depends upon the characteristics of the earth’s orbit around the sun. Over periods of many thousands of years, changes in the earth’s orbit round the sun have led to ice ages and inter-glacial periods. Three factors come into play, preccession of the earth’s orbit, change in the tilt of the earth’s axis to the plane of the orbit and “wobbles” in the tilt of the axis. The fastest of these, and with the smallest effect, is the last with a period of around 45,000 years. In the context of what has happened over the past 100 or even 1000 years, these effects are too slow to have a measurable effect.
Over these periods the emission of heat by the sun is virtually constant. Cyclic changes in the amount of heat emitted by the sun – the quasi 11-year sunspot cycle is the best known. The effects of this are fairly small and swamped by other effects.
Changes in how heat drives the atmosphere
The atmosphere is transparent to short wave radiation – mainly at wave lengths of visible light and UV. The earth absorbs that heat and radiates in the infrared. That outgoing infrared radiation is absorbed and emitted by greenhouse gases, primarily water vapour (H'2'O), carbon dioxide (CO2), methane (CH4), various industrial gases and ozone (O3). Infrared radiation is also absorbed and emitted by cloud and aerosols. The latter come naturally from volcanoes, from dry land and outer space. Man inputs aerosols from industry and other activities. Overall, aerosols have a net cooling effect. Cloud is rather more complex and effects depend on the type of cloud, its height and thickness.
There are many feedbacks; examples are -
- Warmer air can hold more water vapour, a powerful greenhouse gas. Warmer air melts polar ice resulting in less heat being reflected back out to space and more absorption of infrared radiation.
- Drier weather can cause desertification leading to more reflection of incoming short wave radiation.
- Wetter weather leads to more vegetation and less reflection. Both processes affect the carbon cycle.
All the processes can, in principle be calculated using the same computer models that predict daily weather. These do not predict specific weather even on yearly scales. They generate a model climatology.
Is it really getting warmer?
Yes, definitely, the climate is becoming warmer. There has been much argument about the validity of data in the early days of instrumental measurements. I will not address those issues here directly but will just point out the at least 5 separate organisations have produced broadly the same results for global average temperature changes over the past 100+ years. These are the UK Met Office, the Japanese Meteorological Agency, the Goddard Institute for Space Studies, the US National Climate Data Center and a group at Berkley.
(Source - http://berkeleyearth.org/land-and-ocean-data/ )
The great variability in the graph of average global temperature is a consequence of the complexity of the global climate system. It has to be remembered that some 90% of the heat in the system is contained in the oceans compared to about 3% in the atmosphere. The remaining heat is contained in the land and the polar regions. The marked and obvious variations in air temperature are, in fact, rather trivial noise in a global context.
A better measure of heat in the climate system is shown in tidal gauge data dating back to the late 19th century.
Sea levels are now rising faster than at any time during the past few 1000 years. See http://ocean.si.edu/sea-level-rise?gclid=CPTf08eA0dICFW0A0wodXV4CmA.
Why do scientist say that increase in greenhouse gases are the cause?
There are several reasons. First, air temperature is highly correlated with CO2 concentrations.
There is an obvious upward trend in greenhouse gases but the rise in global temperatures is much more variable. This leads to understandable doubts and is a factor used by those that doubt that man is affecting the climate.
An example of the use of statistics in studying climate change has been given in an American Statistical Society publication.
A correlation coefficient of 0.857 from a population of 32 shows that the null hypothesis has a probability of less than 0.001.
How do we know that increases in CO2 are due to man?
First, there is circumstantial evidence comparing the amount of CO2 in the atmosphere with the amount of fossil fuels used since the start of the industrial revolution.
More direct evidence comes from isotope analysis of CO2 in the atmosphere. This shows conclusively that man is largely the cause of increased greenhouse gases. See https://www.esrl.noaa.gov/gmd/outreach/isotopes/.
Climate models are far from perfect for several reasons. However, results from a large number of model runs by many different organisations have been collated by the IPCC in their 4th report.
This is taken from the IPCC Panel 1 Assessment Report 5, Chapter 9 (2013), this shows how all the climate models were able to represent what actually happened since 1870.
This show results for one model run by the US some years ago.
The models can be run with various scenarios and the sensitivity of the climate to different factors estimated. The amounts of carbon dioxide, methane, sulphur dioxide and other gases can be varied and the different responses calculated and compared with observation.
Taken from IPCC Panel 1 Assessment Report 4, Chapter 9 (2007), This shows the results of climate simulations when anthropogenic greenhouse gases are not included. The solid black line shows how air temperatures have actually changed since 1900. The blue lines show model simulations with the average shown as a solid blue line.
From the above, it is quite clear that increases in global average temperature since the industrial revolution are due, in large part, to man’s activities.
How can predictions be improved?
Climate models have been run under different scenarios of human activity. This has given a range of possible outcomes usually shown in diagrams like this. - climate-scenarios.png
Models can be run with more variants of the scenarios. However, there is scope to improve models but this will require further research to model better effects of feedbacks, especially cloud.
Operational weather forecast models are run as ensembles. The initial conditions are varied slightly to account for uncertainties is the data analysis. A similar technique can be used with climate models, see http://www.climateprediction.net/climate-science/climate-ensembles/ .
What should we do about climate change?
The first priority must be to improve understanding of the working of the climate system. Climate models have been shown to replicate climate pretty well in general terms although there are some factors that may never be well modelled. ENSO and other features create swings in climate but do not, in themselves, contribute to climate change in the long term. Particular problems with models are related to feedbacks and the role of cloud is a major, yet unresolved, issue.
Government actions really come down to managing the uncertainties inherent in the science. There are no clear cut answers and much will depend on assessment of impacts.
I am far from competent to make any categorical statements. Common-sense suggests (to me, at least) that governments and planners should take the most likely range of scenarios and act accordingly. They should then invest in improving the science and modify actions and plans accordingly.