Climate Change - Fact or Fiction?

Meteorology is a very broad subject and I should like to make it quite clear that I am not an expert on global warming but, in my Met Office days I was in close working contact with those in the field, some of whom are still involved. This article has been produced largely, and unashamedly, using output from the Hadley Centre for Climate Prediction and Research, now regarded as a world leader on this most important topic.

The Centre, which is part of the Met Office, provides a focus in the United Kingdom for the scientific issues associated with climate change. The main stated aims of the Hadley Centre are:

• To understand physical, chemical and biological processes within the climate system and develop state-of-the-art climate models which represent them;

• To use climate models to simulate global and regional climate variability and change over the last 100 years and to predict changes over the next 100 years;

• To monitor global and national climate variability and change;  

• To attribute recent changes in climate to specific factors;

• To understand, with the aim of predicting, the natural variability of climate

The Hadley Centre was inaugurated by Mrs Thatcher – one of her more enlightened acts – very much on the prompting of Sir Crispin Tickell, Permanent Secretary of the Overseas Development Administration (1984-87), British Permanent Representative to the United Nations (1987-90) and Chairman of the International Institute for Environment and Development (1990-94). He was the author of Climate Change and World Affairs (1977 and 1986) and contributed to many other books on environmental issues including human population and bio-diversity.

Being part of the Met Office, the Centre benefits by having access to the largest and most powerful computer complex in the world used exclusively for meteorology. There are two way benefits in that the physics and mathematics of climate modelling and weather prediction have much in  common. There is a significant economy of scale. 

The Centre employs around 100 staff, some of whom are on short term contract and there is a visiting scientists scheme so that some of the best in the world work there from time to time. It is a multidisciplinary organisation bringing together all aspects of the environment in order to help understand the actions and interactions that occur. There are good and fruitful working links with Universities and with other institutions involved in environmental work.

The centre, itself, is funded by the Department for Environment, Food and Rural Affairs (DEFRA), other United Kingdom Government departments and the European Commission. It has no political axe to grind and no pressures are placed upon it to massage findings in any way. This should be borne in mind when studying its results and comparing them with those of other groups funded by pressure groups with vested interests. Few scientists deny that the climate is changing. Those who have particular commercial interests are prone to downplay the impacts and the probable causes..

Is climate changing? The answer must always be "Yes". Climate has always been changing. But, the question then becomes "Are the current changes really unusual?" and "Is man to blame?" Those seriously in the business of climate change, such as the scientists at the Hadley Centre would say, with true scientific caution, that all the indications are that man is having an adverse effect. The next question is "What will it mean for us?"

Early attempts to determine the role of man in causing climate change used temperature observations and related these statistically to the use of fossil fuels. The qualitative argument is that

short wave radiation from the sun passes through the atmosphere (as it does through a glass window) and heats the earth which, in turn, warms the atmosphere. The earth loses heat by long wave radiation (infra red) and some of this heat is absorbed by the atmosphere due to the presence of certain gases, especially carbon dioxide, CO₂ and various other gases, for example methane, CH₄. Changes in the amounts of these, and other, gases, natural and man made, can change the heat balance of the earth. Although by no means a precise analogue, this is known as the greenhouse effect.

The statistical results provided strong evidence that increases in greenhouse gases might be the major cause of surface and lower atmospheric warming during the 20th century. But, like most statistical statements, it is always necessary to understand the reasons behind them. In order that findings, which may be unpalatable to some, carry weight and lead to action they should be scientifically sustainable. In other words, can our understanding of the physical and chemical processes of the atmosphere really account for the observed changes and can they help to predict the future?

The concept that use of fossil fuels causes global warming is simple to understand, but the reality, like life, is not so simple. Other gases such as CH₄ are also powerful greenhouse gases and can be produced both naturally and as a result of agricultural practices. Man also releases aerosols into the atmosphere, such as sulphate particles and aerosols also enter the atmosphere from volcanic eruption and for other natural reasons. The particles both absorb and radiate heat but their net effect is to cause cooling. The oceans and vegetation absorb CO₂ but bare soil can release CO_2. The way in which these effects take place depends on the temperature. Thus, there are complex actions, inter-actions and feedbacks both   positive and negative.

Any attempt to understand the reasons behind the statistics has to be able to quantify all these effects in the light of an ever changing world economic and social environment. Climate modelling is not just a matter of understanding the workings of the atmosphere. It must also be able to explain and take account of the effects of man on his habitat and, conversely, those of the changing habitat on man.

Climate change will involve more than just temperature. Winter atmospheric surface pressure for the last 50 years show increases over the subtropical North Atlantic Ocean, southern Europe and North Africa. Decreases in surface atmospheric  pressure occurred over the polar

 regions and the North Pacific Ocean. These effects have implications for storm tracks and the frequency of strong winds. A viable greenhouse gas theory has to account for these observed trends. Again, what are the implications for the future?

Global warming will cause sea levels to change. This has very serious consequences for Indian ocean islands in particular. Land reclamation schemes could be seriously jeopardised and the effects of the slow tilting of land masses may be more marked than previously expected.

 weather, particularly temperature,    is to eliminate the effect of changes in land use and other local human activity. There are ways of making allowance for these factors but it is all too easy for the deductions from the data to be discounted. Observations over the oceans do not suffer from these drawbacks although, even here, the derivation of climatological data capable of rigorous scrutiny is by no means straightforward.

The number of ships providing weather data has greatly increased over time and observing practices have changed. A major project in the Met Office, over many years, studied the effects of changing measurement techniques and of varying tracks of ships. Practical experimentation and statistical techniques were used in combination. These data, have been used to derive a graph of changes in sea surface temperature since the start of the data series.

Studies such as this show a warming which could be within the normal noise level of climate variability. The period is too short to make any scientifically defensible deductions about the long term trends. Although they confirm the concept of global warming they might just be showing a large, long period oscillation.

To get longer data records is a problem that has been tackled using "proxy" data. These rely upon information derived from a variety of sources such as width and density of tree rings, sediment or ice cores and fossil remains. Archaeological digs in human settlements can tell us about what crops were grown or what wild plants they used. These are good pointers to the climate of the time. Ice and sediment cores can tell us not only about the prevailing climate but also the constituents of the  atmosphere. Fossil remains can give information on animals and plants at different times in history and, again, provide valuable climate indicators.

Palaeoclimatology, the interpretation of the data is a science in its own right but there are large error bars. Some verification can be obtained back to about 1000 BC due to documentary records kept in China. The totality of techniques is rather like a three dimensional jigsaw of inestimable complexity.

This figure shows, from AD 1000 to 1999, a Northern Hemisphere temperature reconstruction (blue) and instrumental data (red). A smoothed version of the temperature series is shown in black. Error limits at the 95% level (grey shaded)

are also shown. The figure is from a report of The Intergovernmental Panel on Climate Change (IPCC).

There is good factual evidence that the 20th century warming trend is continuing. The average global temperature over land in 2002 was around 1 °C warmer than at the end of the 19th century. This fact, in itself, may not be too significant looked at on time scales stretching back over millennia. However, the climatologists have been able to show that the rate of change is highly statistically significant and not likely to be attributable to normal climate variability.

To determine whether these trends are due to the activity of man, the scientists use mathematical models broadly similar to those used in day to day weather prediction. The differences are that they have to take into account atmospheric chemistry and the effects of changing ocean and land surface characteristics. These are relatively slow processes and not relevant to day to day forecasting. The computations are, very obviously, open to criticism that they are too simplistic to be able to explain what is happening let alone predict it.

The modellers and the climatological statisticians take a number of important steps. First, they compare the behaviour of their models with what has happened and is happening in the period for which we have good instrumental, meteorological data. In other words do the models describe the weather of the last century reasonably well?

Then the models can be checked against longer period records, mainly of temperature, such as the CET series. The next test is to use the proxy data describing the chemical composition of the

atmosphere in previous ages. These data can be put into the models to see if they are capable of producing reasonable representations of the climate as it was both during man's lifetime and in prehistory.

Weather varies naturally from day to day, month to month, year to year, and on longer time scales. Occasionally this variability leads to extremes of temperature or precipitation. An important test of a climate model is whether it can reproduce, credibly, the observed natural variability. A climate prediction index has been developed by the Hadley Centre as a means of comparing the performance of climate models against a range of different indicators (such as surface temperature, rainfall and surface pressure) using a single number.

In a very specific sense, the models are validated by comparing simulations with events such as the last glacial maximum (the peak of the last ice age, around.21000 BC). Such testing gives confidence in predictions of the global mean temperature rise now expected over the 21st century. This is

expected to be in the range 2 - 4.5°C depending on the greenhouse gas emissions assumed by the IPCC

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.

Other possible impacts can be studied such as the response to solar intensity changes and man made aerosols. Volcanic eruptions can be included and compared with observations after such events. These effects, generally, are weaker in terms of long term effects and, therefore, more uncertain than those of greenhouse gases. The past 1,000 years are a particularly important time-frame for assessing the background natural variability of the climate. The solar input has remained largely unchanged and the spatial extent of large-scale climate change during the past millennium can now be estimated with some confidence.

The main observational conclusion is that the variations and trends of all the main indicators consistently and very strongly show an increasing global surface temperature over at least the last century, although substantial shorter-term global and regional deviations from this warming trend are very likely to have occurred

For example, the best estimate of global surface temperature change is a 0.6°C increase since the late 19th century with a 95% probability range of 0.4 to 0.8°C. It is likely that there have been real differences between the rate of warming in the troposphere and the surface over the last twenty years, which are not fully understood. Palaeoclimatological analyses for the last 1,000 years over the Northern Hemisphere indicate that the magnitude of 20th century warming is likely to have been the largest of any century during this period.

The Hadley Centre computer models are run under various assumptions of the chemical composition of the atmosphere. One of these assumes that future emissions of greenhouse gases will follow a "business as usual" scenario of mid-range economic growth but no measures to reduce greenhouse-gas emissions. Other scenarios assume a range of global strategies to reduce dependence on fossil fuels. However, predictions from climate models are always subject to uncertainty because of limitations on our knowledge of how the climate system works and on the computing resources available. Different climate models can give different predictions although, by and large, the differences are not too significant. The ultimate objective of the United Nations Framework Convention on Climate Change is to achieve ". . . stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous interference with the climate system."

The consensus of climate models is that, as future atmospheric carbon dioxide concentrations increase, due to fossil fuel emissions and deforestation, the temperature of the planet will also increase. But, one of the difficulties with trying to simulate changing climate using models is the two way interaction between the carbon cycle and climate. The next diagram shows (the blue line) changes in CO₂ when it is assumed that absorption by vegetation and the oceans with release of CO₂ by soils will remain at the current rates. However, these processes are affected by temperature, by the amount of vegetation, by the amount of CO₂ already absorbed. Changing climate will change sea temperatures and this affects absorption by the oceans. Absorption of CO₂ by vegetation depends upon the temperature, the amount of vegetation and the amount already absorbed. The red line shows the result of calculating using this more realistic scenario.

The patterns of pressure change simulated by climate models for the past century have many of the same features as the observations, but the magnitude of pressure change is much smaller. This implies that climate models underestimate changes in surface atmospheric pressure.

It should be noted that the sea level is expected to continue to rise beyond this period. On this time scale the major component of the sea-level rise is the thermal expansion of the oceans. Because the oceans have a large thermal capacity they take a long time to adjust to changes in atmospheric greenhouse gas concentrations, and the effects on future sea-level rise beyond 2100 (expressed as a

Africa, Australia and Europe.

The modelling study investigated the historic impact on the climate system of greenhouse gases alone; the combined effect of sulphate aerosol, lower atmosphere and stratospheric ozone; the combined effect of volcanoes and changes in the output of the sun.

Whether or not man is responsible for climate change, the question that sailors will be concerned with is what is happening to sailing weather. Measurements from a number of sites across the United Kingdom and Iceland show that the occurrence of deep depressions, which can cause severe impacts, has changed over recent times. However, this storm record is not long enough to determine whether this represents a long-term trend.

Hadley Centre scientists have recently studied changes in storm characteristics over the past 50 years or so. The average number of storms shows a significant increase in the United Kingdom winter period (October to March). Regional analysis shows that the largest increases occur over the southern UK. Iceland has experienced a slight reduction in the number of storms (between October and November), although this reduction can not be separated from natural variability with any degree of certainty.

A reduction in storm frequency in the north and an increase in the south is consistent with a southerly

 movement of the North Atlantic storm track. The severe storms over the UK are more related to strong local gradients of pressure than to the large-scale pressure differences over the Atlantic. One effect that may be starting to become noticeable is the length of the hurricane season. Hurricanes need, among other things, sea temperatures over large areas of at least 26 C. A warming of the seas on the large scale might mean an extension of the season. The 2003 hurricane season started around 22 April

while normally, hurricanes are expected from June onwards.

Another indication of possible global warming effects is the hurricane seen here to be forming over the South Atlantic on 26^th March, 2004. This is the first hurricane ever reported over that ocean. Conventional wisdom is that the seas are just not warm enough. It would be facile to say that this is definite proof of global warming. But, it is just another abnormal occurrence that lends credence to the concept.

It is important to place these results into context. Evidence of storm frequency from daily indices and measurements of wave heights suggest that although it has increased in recent times, the magnitude of storminess at the end of the 20th century was similar to that at the start. This could mean that natural variations in the magnitude of storminess on time scales of several decades or more are responsible for all or part of the trends seen in these new results and that data covering a longer period are needed in order to distinguish a climate change trend from the natural variability.

very green but will be costly to harness. Tidal barrages, again, are very green but are not to everyone's taste. Fuel cells have promise. Hydro-electric is very green but only available in selected areas. It is just possible that implementing some or all of these will reduce the effects and in the very long run, even reverse them.

This is not the forum to indulge in political discussion or to pontificate on what man should do collectively. It may or may not be too late to do anything to change the trends in any significant way

in the  lifetime of our children or grandchildren. The question that the politicians have to ask themselves is "Can we afford to ignore the warnings from the experts and continue to do little or nothing to try to ameliorate what seem to be the logical outcome of the current observed effects?" But, implementation of many sources of green energy will mean that many of us will have to accept unpalatable solutions. Do we have the strength of character to accept restrictions to our personal freedom and reductions in our quality of life for the sake of future generations?

Answers on a postcard, please!

Postscript - Over simplification!

My suggestion that warmer oceans will make hurricanes and typhoons more frequent may be an example of the danger or over simplification. This is a hazard in meteorology. Nothing is simple. Some recent research has suggested that a consequence of warmer oceans may be greater wind shear in the vertical. Tropical storm formation can be inhibited by large wind shear.