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 at: http://www.metoffice.gov.uk/research/hadleycentre/pubs/brochures/

2. ‘The Discovery of Global Warming’ - AIP website
Editor: Professor Stephen Weart, American Institute of Physics
Link at: http://www.aip.org/history/climate/index.html

3. ‘Global Warming – The Complete Briefing’.
Author: John Houghton, ex-Chief Executive UK Met Office
CUP third edition (paperback 2004)

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 inherently a large and complex system, subject to many influences and essentially chaotic in its behaviour, and has experienced many major perturbations during this almost unimaginably long period.

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 became more stable, human populations increased and agricultural techniques were developed, leading to the advanced civilisations of today.

If no significant human influence were present, a gradual temperature decline into a new Ice Age would be inevitable. However, man’s prolific generation of greenhouse gasses is causing global temperatures to rise towards levels not seen for many millennia.

GLOBAL HEAT BALANCE

In global terms, incoming solar radiation is balanced partly by direct reflection from clouds and the remainder via outgoing long wave (infrared) radiation from surface heating. If the Earth’s average surface temperature rises, radiation of heat outwards from the cold 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 several million years the actual average figure has been + 15° C, due to the blanketing effect of water vapour, carbon dioxide and some other trace gasses also present at these upper levels. This 21° C increase is generally termed the ‘natural’ greenhouse effect. 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.

GREENHOUSE GASSES

Water vapour and clouds have a powerful effect upon surface temperatures. Low cloud blocks much of the incoming radiation from the sun, causing localised cooling. However, its presence also limits outgoing radiation, thus locally reducing heat loss from the surface at night. High cloud has only a weak blocking effect on outgoing radiation, as there is usually little water vapour present at these altitudes. Thus far there is little evidence that human activity has affected global climate by increasing general humidity levels or cloud cover, although more extreme weather events appear to be increasing in frequency.

Carbon dioxide, the most important additive greenhouse gas, is currently responsible for about 60% of the enhanced greenhouse effect. 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 greenhouse effect. The preindustrial level was about 0.8 ppmv, but its concentration is now nearly 2.0 ppmv, significant because this gas has a GWP of 28. However, 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 very 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 Gt, gigatons of C) are:

Land (Inorganic C in rocks) 65,000,000 - including 10,000 in fossil fuels
Oceans (Mainly CO2) 40,000 - including methane hydrates
Soils (Organic C) 2,000 - including soil litter, peat bogs
Atmosphere (99.6% CO2) 750
Land plants (Organic C) 600

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

Respiration (land plants and animals) + 50 Gt/year (to atmosphere)
Respiration (decomposition, burning) + 60
Photosynthesis (land plants) - 112 (from atmosphere)
Oceans (outgassing, zooplankton) + 90
Oceans (absorption, phytoplankton) - 92

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 reduces the effect of an input stimulus, whereas positive feedback reinforces the effect. One key example of negative feedback from global warming is increased plant photosynthesis and carbon fixing from phytoplankton growth, resulting from higher local concentrations of CO2. However, such beneficial effects weaken and then disappear as temperatures in a particular ecosystem climb beyond a certain point.

In contrast, a highly topical example of positive feedback is the climatic effect of albedo change due to polar melting. The disappearance of highly-reflective ice cover exposes dark ground to solar heating, which in turn encourages further melting. Large-scale loss of ice cover will reach a critical ‘tipping point’ for that area. Beyond that point the area becomes ice-free and will remain so unless prolonged cold conditions return and produce substantial and regular snowfalls. Greenland appears likely to suffer that fate.

A number of other positive feedbacks affecting CO2 have been identified. Increased temperatures cause higher respiration of CO2 by soil microbes and zooplankton in the oceans. Continued warming will also affect rainforest growth and could cause other vegetation to die-back due to climate stress or 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. Because feedback processes may be responsible for 60-70% of any predicted temperature rise and these processes themselves are not wholly predictable, such levels of uncertainty are of great concern.

Methane feedback effects are also being identified. Significant quantities of methane are now being released, due to permafrost melting and large-scale drying out of northern wetlands. Finally, methane hydrates exist in very large quantities in cold sediments on continental shelves and these could be vulnerable to rising temperatures. Fortunately, deep-water temperatures should not rise to a dangerous extent during the present century.

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 warning of the risks of ‘dangerous climate change’. We are so used to seeing incessant small variations in weather, almost always fluctuating within a tolerable local range, that we cannot 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 generally benign and stable conditions; significant and irreversible climate change could conceivably destroy it. Effective action is therefore most urgent.

Many individual scientists, engineers, business managers and ordinary citizens now fully appreciate the need for determined action, and almost all major political and administrative organisations worldwide 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 required resources have been estimated (Stern Report 2006) at approximately 1% of global GDP. This estimate was supported by work in the IPCC, but has been criticised as an underestimate. Nevertheless, current national military expenditures worldwide are broadly of this order. Even so, the problems involved in negotiating and implementing the necessary policies on a global scale, taking into account conflicting social, commercial, political and national interests, are very considerable and these are bound to cause significant delays.

If emissions continue to rise and action is delayed, both the magnitude of the ultimate problem and the levels of potential risk will grow significantly. If no action were taken, Stern estimated for the worst case that there could be an economic loss of 40% Global GDP potentially achievable by the end to the century. This serious position would then probably deteriorate further beyond 2100. It is therefore not acceptable either to think we can live with the effects of warming as they arise or to contend that steady growth in global GDP will allow our great-grandchildren to shoulder most of the burden.

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.


Page Information

  • 3 weeks ago [history]
  • View page source
  • You're not logged in
  • No tags yet learn more

Wiki Information

Recent PBwiki Blog Posts