SCIENCE OF CLIMATE CHANGE – A BRIEF REVIEW

 

 

INTRODUCTION

 

The science of climate change is a vast and extremely complex subject. This review merely aims to set out some key points as a brief introduction. The reader can easily accumulate further relevant information by internet search, but the following are useful references:

 

1. ‘Climate change and the greenhouse effect’. Editor: Professor John Mitchell, Chief Scientist UK Met Office.

Co-publication by DEFRA/Met Office (Hadley Centre) 2005. Link via: http://www.metoffice.gov.uk/climate-change/guide

 

2. ‘The Discovery of Global Warming’. Editor: Professor Stephen Weart, American Institute of Physics

Link at: http://www.aip.org/history/climate/index.html

 

3. ‘Global Warming – The Complete Briefing’. CUP third edition (paperback 2004)

Author: John Houghton, ex-Chief Executive UK Met Office

 

4. The US National Oceanic and Atmospheric Administration (NOAA) has been monitoring atmospheric greenhouse gases for over 30 years and reports its Annual Greenhouse Gas Index at: http://www.esrl.noaa.gov/gmd/aggi/

 

5.  NOAA also publishes an annual statement on the state of the global climate for the Bulletin of the American Meteorological Society. Recent statements can be found at:  http://www.ncdc.noaa.gov/bams-state-of-the-climate

 

6. 'Sustainable Energy - without the hot air' - ISBN: 9780954452933 / 978-1-906860-01-1

A popular science book by David JC MacKay, Professor of Natural Philosophy, Department of Physics, University of Cambridge.

Also available for free download over the internet at: http://www.withouthotair.com/download.html

 

 

CLIMATE CHANGES IN THE PAST

 

All living organisms depend for their existence on stable conditions within the biosphere, a thin layer extending from shallow depths underground, rather deeper in the oceans, for about 5 miles upwards into the atmosphere. Conditions here have remained broadly stable for several thousand million years. However, the atmosphere is a large and complex system, subject to many influences and essentially chaotic in its behaviour, and it has experienced many major perturbations during this almost unimaginably long period. James Lovelock has pointed out that, since its early appearance on the planet, life itself has had a crucial impact on the atmosphere's constituent gasses and thus the behaviour of the biosphere.

 

Some geological eras have been extremely cold by modern standards, with glaciation spreading to lower latitudes and reduced sea levels. In other periods temperatures have been higher than now, with greater concentrations of CO2 and oxygen, and much reduced ice cover near the poles. Ocean currents have been radically redirected by the reshaping of land barriers through tectonic plate movement. Periods of extensive and prolonged volcanic activity and very large meteor strikes have also affected the biosphere.

 

Over the past million years or so climatic conditions have oscillated through a series of Ice Ages and Interglacials, triggered by regular variations in the Earth’s orbit round the Sun, reinforced by positive feedback from changing CO2 levels and ice sheet cover. During the most recent Ice Ages emerging groups of humans led a hunter-gatherer existence in variable, often very harsh climates. Over the last 10,000 years conditions in the current Interglacial have become noticeably more stable; human populations have increased following the development of agricultural techniques, ultimately leading to the advanced civilizations of today.

 

At the present time a gradual temperature decline into a new Ice Age might have been expected. However, the effects of early agriculture in the Bronze Age and the subsequent rise of modern technologies have halted and then sharply reversed this trend. Our prolific generation of greenhouse gasses is now causing global temperatures to rise towards levels not seen for many millennia, with potentially dangerous consequences.

 

GLOBAL HEAT BALANCE

 

In global terms, incoming solar radiation is balanced partly by direct reflection from clouds and the remainder through outgoing long wave (infrared) radiation from surface heating. If the Earth’s average surface temperature rises, radiation of heat via the colder upper levels of the atmosphere into space has to increase until a new balance is reached.

 

If the atmosphere were a simple mixture of oxygen and nitrogen, the global average temperature would be – 6° C. However, for much of the Earth's history the actual average figure has been + 15° C, due to the blanketing effect of water vapour, carbon dioxide and other trace gasses also present at these upper levels. This 21° C increase is generally termed the ‘natural’ greenhouse effect. However, evidence of an ‘enhanced’ greenhouse effect due to the impact of human technology and agriculture on the atmosphere is now clearly visible in global temperature records. A recent analyses has been published by the UK Met Office comparing the long-established UK time series with US data from NASA and the NOAA; the three independent series show remarkably close agreement: http://www.metoffice.gov.uk/climatechange/science/explained/explained5.html

 

GREENHOUSE GASSES

 

Water vapour and clouds have powerful but variable local effects upon surface temperatures. Lower-level clouds reflect or block much of the incoming radiation from the sun, producing cooling effects on the area immediately below. However, its presence also limits outgoing radiation, which would reduce stored heat loss from the same area at night. High-level cloud has only a weak blocking effect on outgoing radiation, and there is little water vapour present at very high altitudes. Rising CO2 levels and hence increased global temperatures are starting to produce a measurable effect on humidity levels. Increases in water vapour then act as a powerful positive feedback, amplifying the direct effects of CO2 (see below) by a factor recently estimated as high as 2. The broad effects of higher temperatures on global cloud cover at lower levels have long been the subject of considerable uncertainty, but recent work suggests that oceanic warming is likely to reduce low level cloud cover over those areas most affected, reinforcing the general warming trend still further. As a separate issue, hurricane and tropical storm activity is likely to become increasingly frequent, due to energy transferred to the atmosphere by warmer surface waters. 

 

Carbon dioxide, the most important additive greenhouse gas, is currently responsible for about 60% of the direct forcing effect due to human activity. Its concentration has risen steadily from the preindustrial level of about 280 ppmv (parts per million by volume) to over 360 ppmv at present; levels are currently forecast to reach 600 ppmv before 2100. Once present, CO2 stays in the atmosphere for periods in excess of 100 years. For comparison purposes, CO2 is given a Global Warming Potential (GWP) index of unity.

 

The second most important component is methane; this gas contributes about 30% of the enhanced direct greenhouse effect. The preindustrial level was about 0.8 ppmv, but its concentration is now nearly 2.0 ppmv. This is a significant effect, because methane has an estimated GWP of 28 (NB. this figure may be revised to 32). As a mitigating factor, atmospheric methane is broken down much more quickly than CO2.

 

Nitrous oxide is a relatively minor, but by no means negligible, greenhouse gas. It is currently present at a concentration of 0.3 ppmv (about 13% greater than the previous natural level). Its importance stems from its high GWP of 297.

 

Ozone has a weak shielding effect at high altitude (due to low concentration) and a significant greenhouse effect at lower altitude, but is particularly important in shielding the Earth’s surface from ultraviolet radiation. Clorofluorocarbon (CFC) pollution of the atmosphere was recently identified as the cause of the so-called ‘ozone hole’ over Antarctica, but firm international action rapidly dealt with this problem. This outcome had implications for climate change, since CFCs are also long-lasting and powerful greenhouse gasses, with extremely high GWPs (around 10,000).

 

 

 

AEROSOLS

 

Naturally occurring aerosols such as low-level smoke from forest fires and persistent high-level dust layers thrown up by volcanic eruptions generally produce a cooling effect.

 

Man-made smoke due to forest or agricultural crop burning will obviously have a similar effect, as do aerosols created by industrial processes, road vehicles, etc. High-level vapour trails in busy air traffic areas also have a measurable cooling effect. Paradoxically, efforts to clean up human emissions have the effect of increasing climate change.

 

 

CARBON SOURCES AND SINKS

 

The planet’s main carbon reservoirs (expressed in gigatons of C) are:

 

Land (Inorganic C in rocks) 65,000,000 Gt - including 10,000 in fossil fuels

Oceans (mainly CO2) 40,000 Gt - including methane hydrates

Soils (Organic C) 2,000 Gt- including soil litter, peat bogs

Atmosphere (mainly CO2) 750 Gt

Land plants (Organic C) 600 Gt

 

Accurate global accounting is difficult, but the main natural exchanges of carbon-bearing gasses (mainly CO2 and methane) are currently estimated as follows (positive signs to atmosphere):

 

Respiration (land plants and animals) + 50 Gt/year

Respiration (decomposition, burning) + 60 Gt/y

Photosynthesis (land plants) - 112 Gt/y

Oceans (outgassing, zooplankton) + 90 Gt/y

Oceans (absorption, phytoplankton) - 92 Gt/y

 

Relatively small amounts of carbon from slow rock weathering also reach the oceans via river flows and dust storms. After lengthy periods surplus carbon becomes stored in subsurface soil, sand and peat deposits, and in ocean sediments from dead marine organisms. Over geological time-scales these deposits are compacted and buried deep in the earth’s crust, but volcanic eruptions ultimately recycle some carbon back to atmosphere.

 

The overall effect is an approximate balance, but with a net extraction rate of about - 4 Gt/year from atmosphere.

 

From the 1850s onwards the cumulative effects of industrialising economies, urbanisation, intensive agriculture and growing fossil fuel burning, amplified by exploding global population, have led to a reversal of the natural carbon flow. Fossil fuel burning generates about 6.5 Gt/year of carbon emissions, plus major land-use changes and biomass burning contributing 1.5 Gt/year, giving total emissions of roughly + 8 Gt/year (into atmosphere).

 

The natural carbon sinks consequently appear capable of balancing roughly half of humanity’s carbon emissions. The excess emissions, about + 4 Gt/year (into atmosphere) constitute the current driver of global warming. This estimate is consistent with the rising CO2 and methane concentrations mentioned above.

 

EFFECT OF EMISSIONS ON GLOBAL MEAN TEMPERATURE

 

The earliest calculation of the effects of doubling CO2 concentration from pre-industrial levels suggested a global mean temperature increase of 5º C, but more modern estimates now produce a ‘first cut’ figure of about 1º C. However, to this initial calculation must be added the contributions of methane emissions, water vapour and the overall effects of positive feedback (mentioned below), leading to an overall temperature increase in the region of 2.5 - 3º C. It does now look very likely that CO2 concentrations will produce such temperatures by about 2100, and temperatures would be bound to increase further during the next century. If emissions accelerate beyond present levels, some estimates predict a temperature rise of 5º C by this date, although the very large thermal reservoir effect of the earth’s oceans does have a braking effect. No safe ultimate temperature level has been identified (in terms of survival of most current biological species).

 

Urgent efforts are now being made to improve the performance and reliability of global climate models, leading to better predictions of overall climate sensitivity and likely changes in regional climates. However, success will very much depend on better knowledge of the physical processes involved and provision of critical inputs such as cloud distribution data from satellite surveys and a greater understanding of key ecological systems.

 

CLIMATE STABILITY AND FEEDBACK

 

Fifty years ago most scientists in the field were confident that self-regulating processes would always maintain the Earth’s climatic system in its current relatively benign state. However, analysts now view global climate as a chaotic system, capable of existing in at least 3 quasi-stable states (Ice Age, Interglacial and Warm Era). Several climate models, reinforced by paleoclimatic evidence such as ice core data, suggest that this is a much more realistic view. Today, many scientists are concerned that rapidly increasing emissions are driving global climate towards a crucial ‘tipping point’, beyond which recovery would become almost impossible and conditions would become increasingly adverse to human civilisation as it exists today.

 

When considering the stability of a dynamic system, negative feedback will reduce the effect of an input stimulus, whereas positive feedback reinforces its effect. One key example of negative feedback on CO2 due to global warming is the increased rate of photosynthesis and carbon fixing by land plants and phytoplankton resulting from higher local concentrations of CO2. However, such beneficial effects weaken and then disappear as temperatures affecting a particular ecosystem climb beyond a certain point.

 

In contrast, a now obvious example of positive feedback is the climatic effect of albedo change due to polar melting. The disappearance of highly-reflective ice cover exposes absorptive open water and ground surfaces to solar radiation, which in turn encourages further melting. Large-scale loss of ice cover may eventually pass a critical ‘tipping point’ for that area. Beyond that point the region affected becomes virtually ice-free and will remain so unless prolonged cold conditions return and produce substantial and regular snowfalls. Greenland's ice cover is rapidly diminishing, as is the extent of Arctic Ocean summer ice. Concerns are also expressed about potential major instability of the Antarctic ice sheets, since their melting would have a dramatic effect on global sea levels.

 

A number of other positive feedbacks affecting CO2 have also been identified. Increased temperatures cause higher respiration of CO2 by soil microbes and zooplankton in the oceans. Continued warming may also reduce rainforest growth and local humidity, affecting other vegetation due to diminishing river flows, and lead to regional desertification. Oceanic absorption of CO2 is appreciably reduced by gradual warming of the surface layers, and possibly stronger winds due to climate change. These are very important feedback mechanisms, due to the large scale of the carbon flows involved. Since feedback processes may generate an additional 60-70% in temperature increase and these processes themselves are not wholly predictable, such levels of uncertainty are of great concern

 

Methane feedback effects are also important. Significant quantities of methane are now being released, due to progressive melting of permafrost and large-scale drying out of northern wetlands. Even more dangerously, methane hydrates exist in very large quantities in cold sediments on continental shelves; these comparatively shallow deposits could be vulnerable to rising temperatures, particularly in the Arctic.

 

Several US agencies are now cooperating to study various mechanisms which, possibly acting in combination, might be sufficiently powerful to stimulate major climatic changes.

 

Note: The RealClimate website includes a recent (2010) paper explaining feedback processes in more depth. Further material is contained elsewhere in this website - An Explanation of Feedbacks .

 

CONCLUSION

 

The scale and impact of humanity’s carbon emissions on the planet’s climate system is now obvious. We cannot be certain of the ultimate effect on the habitability of the biosphere, but well-qualified scientists are now concerned about the risks of sudden and dangerous climate change. We are so used to seeing incessant small variations in weather, almost always fluctuating within a tolerable local range, that we do not fully comprehend the potential results of a drastic and permanent shift in climate. The unprecedented scale and complexity of our technological civilisation largely depends on the maintenance of the generally benign and stable climatic conditions which have persisted throughout the last 10 millennia. Significant and irreversible climate change could make any confidence in a secure and civilised existence by billions of human beings extremely problematic - some might say impossible. 

 

Many individual politicians, scientists, engineers, business leaders and ordinary citizens now fully appreciate the need for determined action. Also, virtually all political and administrative entities acknowledge, at least in principle, this same urgency. A wide range of promising technologies are either available or are potentially deployable, given determined effort and adequate investment. The Stern Report in 2006 estimated the required resources at approximately 1% of global GDP and this figure has been broadly supported by work reported within the IPCC. For comparison, current national military expenditures worldwide are broadly of this order. Investment on this scale is therefore clearly affordable in principle. Even so, the last IPCC meeting in Copenhagen highlighted the problems involved in negotiating and implementing the necessary policies on a global scale, taking into account conflicting social, commercial, political and national interests.

 

If emissions continue to rise and action is delayed, both the magnitude of the problem and the possibility of passing an irrecoverable tipping point will grow significantly. Stern estimated for the worst case that there could be an economic deficit of 40% Global GDP  for 2100 (ie. 40% reduction from the figure potentially achievable by that date); this is a huge loss. Further unstoppable decline beyond 2100 would seem highly likely. Arguments that we can simply compensate for the effects of warming as they arise, or that continued growth in global GDP will allow our great-grandchildren to easily shoulder most of this burden, are simply not credible.

 

The Historical Record

 

This appendix reviews the chronology of how the anthropogenic influences on climate change were discovered and the international structures for scientific and intergovernmental cooperation developed.