An Engineers Appreciation of the Science Behind Global Warming
 Introduction
This engineer (Jeff Temple) believes that the most important consideration to this highly emotional debate, is that all discussions should be based on science, with politics excluded. I have seen so many times strange and erroneous arguments and opinions, put out as facts (see some of the exchanges on this site as examples), that the average person could be forgiven if they were confused as to whether there was global warming and climate change, or not.
This short area of this site attempts to give an overview of the information on Global Warming I have learnt, and which I thought might be of interest to other engineers, without having the need to search the Internet, or buy a book. This section starts with data gathering, and then goes through a review of the science. It examines the causations and resulting effects, and details some of the checks and balances to make sure that the conclusions are indeed valid. This is necessarily just a very short look at climate science, and skims over many important issues. However, I do hope it will be useful for my fellow engineers. I am in the process of putting these pages together, and would welcome assistance from any engineer who felt able to contribute to better understanding of the problem, either through corrections or additions. Needless to say the subject is extremely complex, so in the present we can just touch on the surface, and many, many aspects are simplified or left off.
Cause and Effect The largest indicator of Global Warming is of course higher temperatures witnessed on our planet. The last decade of the 20 th Century was the warmest of the entire global instrumental temperature record, starting in the mid-19 th century. All 10 years rank among the 15 warmest and include the 6 warmest years on record.
The only possible cause of the increased temperatures found to date is increased greenhouse gas concentrations, resulting from industrialisation. I will now attempt to take each topic on its own.
How do we know the CO 2 levels are increasing? The Mauna Loa observatory, operated by the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (1) , has been measuring carbon dioxide and other gases in the air since 1958. It is about 3,350 metres above sea level on the second-highest mountain in Hawaii . This location means its measurements are of some of the cleanest air on Earth.
The Mauna Loa results (see below, Figure 1) showed CO 2 increasing by about 1ppm per year at the start of the period of results gathering, whilst today that increase is about 1.8ppm./year, ie the increase is gathering speed. A side note. The black line represents the yearly average. The red line shows the true CO 2 concentrations, with the rise and fall caused by the seasonal difference between the hemispheres, with the northern hemisphere having a larger land mass, and hence potential to absorb CO 2 in its growing season.
Figure 1: Atmospheric Carbon Dioxide - Mauna Loa
As confirmation for the source, the CO2 from the burning of fossil fuels is isotropically different to the background CO 2 , so this link to increased CO 2 concentration can be proven.
Now however comes the sting in the tail. The principal effect from the increased temperature has been an increase in water evaporation, which is also a potent GHG. So a positive feedback loop is created, forcing our planet's temperature to even higher levels.
Figure 2: CO 2 Concentration in ice core samples
There are now several other stations dotted around the world, which are all confirming the results seen in Muana Loa. But what about before these stations were started? We do have rather random results going back a couple of centuries where scientists made analyses, but for more systematic results we go to ice core samples, which can be dated from isotopes, and where the air bubbles release their fund of information on what our climate was like going back over 400,000 years. The results from a collection of different sources can be seen on the IPCC site (2) in a fascinating plot showing variations of temperature, methane, and atmospheric carbon dioxide concentrations derived from air trapped within ice cores from Antarctica . These s amples, looking back before the industrial age, and therefore before extensive use of fossil fuels, show the concentration of carbon dioxide in the atmosphere stood at about 280 parts per million. From the beginning of the industrial revolution (about 150 years ago) when we started burning fossil fuels in large quantities, CO 2 concentration has taken off.
Another source of information for the massive and recent increase in CO 2 is from ice core samples, such as the curve shown left in Figure 2 (3) , which were taken from the Law Dome, in East Antarctica, where there has been negligible melting of the ice sheet surface, low concentrations of impurities, regular layering undisturbed at the surface by wind or at depth by ice flow, and high snow accumulation rate. The ice cores were dated by counting the annual layers in oxygen isotope ratio ( δ 18 O in H 2 O), ice electroconductivity measurements (ECM), and hydrogen peroxide (H 2 O 2 ) concentrations. For these three parameters, each core displayed clear, well-preserved seasonal cycles allowing a dating accuracy of ±2 years at 1805 A.D. for the three cores and ±10 years at 1350 A.D.
Global Temperature (4)
Today's scientific activity relating to climate and weather is co-ordinated on a global scale. One hundred and eighty-five countries participate in this program, and between them they monitor 10,000 land based observation stations, 7,000 ship-based ones, and ten satellites. Absolute estimates of global mean surface temperature are difficult to compile for a number of reasons, since some regions of the world have few temperature measurement stations (e.g., the Sahara Desert ), interpolation must be made over large, data sparse regions. In mountainous areas, most observations come from valleys where the people live so consideration must be given to the effects of elevation on a region's average as well as to other factors that influence surface temperature. The key in collecting this information is therefore not to find a “Global Average Temperature”, but to identify any deviation from historical averages, looking at a sufficiently long time (if we select a short period, the information will be overly dependent on the starting conditions, and potentially upset by “noise”). By looking at deviations from long term averages, this means that the results are not strongly affected by the addition or removal of data from a location where average temperatures are very high, or low.
Measurement by Thermometer
The earliest records of temperature measured by thermometers are from western Europe beginning in the late 17 th and early 18 th centuries. The network of temperature collection stations increased over time and by the early 20 th century, records were being collected in almost all regions, except for Polar Regions where collections began in the 1940s and 1950s. Two widely recognized research programs have used the available instrumental data to reconstruct global surface air temperature trends from the late 1800's through today, and can be found in reference 4. Both use the same land-based thermometer measurement records.
Measurement by Balloon (radiosondes)
The longest data sets of upper air temperature are derived from instruments carried aloft by balloons (radiosondes). The radiosonde data set became global in about 1958.
Measurement by Satellite
Satellite measurements have been used to reconstruct global atmospheric temperatures since 1979. Satellites do not measure temperature as such, but measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.
Proxy Measurements
We are not limited to direct measurements to have a source of historic temperature change, as proxy measurements can do this for us. Proxy measurements can include Tree-rings, Ice Cores, Corals, sediments, glacier lengths, plus several other more obscure methods. These have been compared in many studies, available on the site referred to below in reference 4).
Conclusions for Global Temperature
Bringing all the above independent sources of temperature information together, the climate scientists have been able to determine common trends through history. The massive amount of data provides one clear indication that the modern earth is warming: that the mean annual surface air temperatures of the earth have risen approximately 0.6°C (1.1°F) since 1860.
Other Independent Sources of Information
Even though sceptics continue to question the Global Warming, it's also worth noting that even if we couldn't calculate a global mean temperature, we still know that climate is changing because we have multiple independent lines of evidence. These include (among many others) the changes we're seeing in ice covered areas of the world, ocean heat content, species ranges, and the timing of key life events such as migration, bud burst, and flowering for a wide range of species.
Final Words on Measurements
Today we are closely monitoring everything we can think of. We are much more able to generate quality reconstructions of the recent past for those factors we did not think of measuring, until now. We know how the sun is behaving. We know when and how hard the volcanoes are erupting. We know the atmospheric levels of ozone, CO 2 , CH 4 , NO 2 , etc. to a high degree of precision, and on a month to month basis, across the globe. We know where the continents are, how the oceans are flowing and the size of the ice sheets.
Consequently, our understanding of the present changes is far superior to any point in the past, and we are also learning more of the past as we move forward, and technologies improve. In short, we are becoming more and more confident of the conclusions we reach.
Greenhouse Effect
The existence of the greenhouse effect has been known about for a long time. In a paper published in 1896 by the Swedish scientist Arrhenius, he discusses the mean temperature of the ground being influenced by the presence of heat-absorbing gases in the atmosphere. These greenhouse gases absorb some of the energy that is emitted from the Earth's surface, preventing this energy from being lost to space. As a result, the lower atmosphere warms and sends some of this energy back to the Earth's surface. When the energy is "recycled" in this way, the Earth's surface warms.
The conditions in our c limate are determined by conservation of energy and the Stefan-Boltzmann radiation law. The Stefan-Boltzmann law as modified for a grey-body radiator is:
F = ε σ T 4
Here,
F is the flux per unit area emitted by the object in question,
ε is the emissivity (a value that can vary from 0 to 1),
σ is the Stefan-Boltzmann constant, and
T is the absolute temperature.
If we change the conservation of Energy, by increasing the retention of energy by the greenhouse gases, then Climate will change. This is radiation physics, pure and simple. Known spectral properties of the CO 2 and other greenhouse gas molecules in the atmosphere allow a relatively accurate estimate of how increased atmospheric CO 2 concentrations affect the radiation budget of the earth system, so the additional radiative forcing of CO 2 that has accumulated in the atmosphere since the pre-industrial era can be relatively accurately estimated.
Arrhenius overestimated the climate's sensitivity to CO 2 by a factor of 2. At the same time, he hugely underestimated the degree of warming, assuming CO 2 would rise very slowly. This is the core to the problem of Global Warming, the increasing amount of Greenhouse Gases, and their properties of holding the heat within our planet.
Climate Models
The info rmation we have available on our climate is input into very sophisticated models which seek to simulate the way that the atmosphere behaves, and to predict how it will behave in the future. These models of the climate are verified against observed changes before being used to make future climate projections. Three of the main validation techniques are (5) :
• comparison against recent change - observations of climate from numerous sites around the globe are available from recent decades whilst some individual records such as the Central England Temperature go back several centuries;
• comparison against observed climate variability - the climate is naturally variable from day to day, month to month, year to year and over longer timescales. Occasionally this leads to extremes of temperature or precipitation, so an important test of a climate model is whether it can credibly reproduce such variability;
• comparison against past climate - climate models can be used to simulate climates of the more distant past, such as the last glacial maximum (the peak of the last Ice Age around 21,000 BC). Model results are compared to evidence of past change, such as tree-ring growth or the thickness of sediment layers in core samples.
Results indicate that the models presently used, such as from the Hadley Centre in UK , meet these criteria with a high level of accuracy (6) .
Validation exercises such as these provide compelling evidence that, at least in terms of gross temperature response, the model is effectively reproducing what has been observed, and this gives us confidence that the models are adequate tools for the prediction of future climates.
The following are examples from empirical results, which were predicted by the models, and this is far from an exhaustive list:
• The rapid increase in atmospheric greenhouse gases should throw the Earth's radiation budget out of balance, because the ocean has not yet had time to warm up to restore balance. The expected imbalance has been observed.
• The planet's energy imbalance has implications for the pattern of subsurface ocean warming. The predicted pattern has been observed.
• Satellite observations indicate that mid-tropospheric water vapour is indeed increasing with temperature, as the theory requires and as models predict.
• Melt-back of Northern Hemisphere sea ice.
• Nearly worldwide melting of mountain glaciers, many of which survived previous naturally occurring warm periods.
• The theory predicts that the stratosphere should be cooling at the same time the surface is warming. This pattern is observed.
Climate vs. Weather (chaotic, or not?)
Many people confuse climate with weather, and thereby think that Climate is chaotic, and by consequence cannot be modelled.
Weather
What we call the weather is a highly detailed mix of events that happen in a particular locality on any particular day - rainfall, temperature, humidity and so on - and its development can vary wildly with small changes in a few of these variables. Weather can be described as chaotic since a look forward depends on the starting conditions, ie it is a dynamic system, and the errors in the starting conditions will diverge the forecast. In scientific terms, the Navier-Stokes equations were firmly established in the 19th Century as the system of nonlinear partial differential equations which describe the motion of most commonly occurring fluids in air and water. Use of these equations gives solutions which provide a vehicle for our computations in fluid dynamics, and which are used in most forecasting models. As Reynolds Number increases, fluids develop vortices, with turbulence at the edges and in mid-stream, the flow structure ultimately going mathematically chaotic. To overcome this chaos, forecasters use a variety of approximations, accurate only for a few days .
Climate
Our climate is a highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and human-induced forcings such as the changing composition of the atmosphere and land-use change. Climate is something akin to the average of the weather, and in this way is not subject to the same “chaos”.
Forecasting
Weather forecasting is dependent on the starting conditions, making predictions based on those starting conditions. These predictions contain small errors, which are amplified as we look forward. A small error at the start can therefore become something large over time (butterfly effect). Climate on the other hand is a sort of average of the weather, and by taking the average we arrive at something that is not chaotic, and which is entirely predictable. We cannot predict with certainty the weather in London or Rome in a week's time, but we can predict with certainty the overall climate in Rome compared to the climate in London , or winter compared to summer, or the climate in a desert compared to a rain forest. In scientific terms, weather is a prediction based on starting conditions which diverge, while climate is a boundary value problem and determined by the boundary conditions, i.e., the incoming solar irradiance and the out-going radiation of the atmosphere (planet earth can be considered like an envelope), therefore, modelling the climate is quite feasible. Climate modelling allows us to look at and understand longer term trends by changing inputs and outputs to the envelope. This confusion between climate and weather is an often cited, but wrong, criticism of prediction of Global Warming .
One of the best ways that we can validate our forecasts, as predicted by the models, is to separate out the various effects, and see how close we come to actual behaviour. The initial point was set back at the start of the industrial revolution, and then three predictions were made, considering change to our climate from only natural processes, from only anthropogenic forcing, and then including both forcings. The results are shown in Figure 3 (7) , and show pretty convincingly that the models have got it right. “ The simulations represented by the band in (a) were done with only natural forcings: solar variation and volcanic activity. Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols, and those encompassed by the band in (c) were done with both natural and anthropogenic forcings included. From (b), it can be seen that inclusion of anthropogenic forcings provides a plausible explanation for a substantial part of the observed temperature changes over the past century, but the best match with observations is obtained in (c) when both natural and anthropogenic factors are included.”
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Figure 3: Climate Model simulation of Climate Change
Conclusions
In forecasting the climate, we do not need to be able to predict the temperature, or whether it will rain, on a specific day 100 years from now, but we do need to know if and how the climate will change, and the models will do just that for us.
References
1. National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html
2. http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/fig2-22.htm
3. Historical CO 2 Records from the Law Dome by D.M. Etheridge et al, http://cdiac.ornl.gov/trends/co2/lawdome.html
4. Prepared using parts from NOAA Satellite and Information Service http://www.ncdc.noaa.gov/paleo/globalwarming/instrumental.html
5. http://www.metoffice.gov.uk/corporate/pressoffice/anniversary/hadley.html
6. Recent Climate Observations Compared to Projections, by Stefan Rahmstorf et al, Science Vol 316 4 th May 2007
http://www.pik-potsdam.de/~stefan/Publications/Nature/rahmstorf_etal_science_2007.pdf
7. http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/figspm-4.htm |
