CLIMATE CHANGE AND HUMAN HEALTH

Human societies over the ages have depleted natural resources and degraded their local environments. Populations have also modified their local climates by cutting down trees or building cities. It is now apparent that human activities are perturbing the climate system at the global scale. Climate change is likely to have wide-ranging and potentially serious health consequences. Some health impacts will result from direct-acting effects (e.g., heatwave-related deaths, weather disasters); others will result from disturbances to complex ecological processes (e.g., changes in patterns of infectious disease, in freshwater supplies, and in food production).

WHAT IS CLIMATE CHANGE?

Global climate change is caused by the accumulation of greenhouse gases in the lower atmosphere. The global concentration of these gases is increasing, mainly due to human activities, such as the combustion of fossil fuels (which release carbon dioxide) and deforestation (because forests remove carbon from the atmosphere). The atmospheric concentration of carbon dioxide, the main greenhouse gas, has increased by 30 percent since preindustrial times.

Projections of future climate change are derived from global climate model or general circulation model (GCM) experiments. Climatologists of the Intergovernmental Panel on Climate Change (IPCC) review the results of these experiments for global and regional assessments. It is estimated that global mean surface temperature will rise by 1.5° to 3.5° C by 2100. This rate of warming is significant. Large changes in precipitation, both increases and decreases, are forecast, largely in the tropics. Climate change is very likely to affect the frequency and intensity of weather events, such as storms and floods, around the world. Climate change will also cause sea level rise due to the thermal expansion of the oceans and the melting of the mountain glaciers. Global mean sea level is anticipated to rise by 15 to 95 centimeters by 2100. Sea level rise will increase vulnerability to coastal flooding and storm surges. The faster the climate change, the greater will be the risk of damage to the environment. Climatic zones (and thus ecosystems and agricultural zones) could shift toward the poles by 150 to 550 kilometers by 2100. Many ecosystems may decline or fragment, and individual species may become extinct. The IPCC Second Assessment report concludes that climate change has probably already begun.

IMPACTS ON HEALTH

To assess the potential impacts of climate change on health, it is necessary to consider both the sensitivity and vulnerability of populations for specific health outcomes to changes in temperature, rainfall, humidity, storminess, and so on. Vulnerability is a function both of the changes to exposure in climate and of the ability to adapt to that exposure (see Figure 1).

Science classically operates empirically, via observation, interpretation, and replication. However, having initiated a global experiment, it would not be advisable to wait decades for sufficient empirical evidence to describe the health consequences. Risk assessment must therefore be carried out in relation to future environmental scenarios. The traditional "top-down" approach is to

Figure 1

answer the question, "If climate changes like scenario X, then what will be the effect on specific health outcomes?" In contrast, "bottom-up" approaches begin with the question, "How much climate change can be tolerated?"

It is important to distinguish between "climate and health" relationships and "weather and health" relationships. Climate variability occurs on many time scales. Weather events occur at daily time scale and are associated with many health impacts (e.g., heatwaves and floods). Climate variability at other time scales also affects health. In particular, the El Niño Southern Oscillation has been shown to influence interannual variability in malaria, dengue, and other mosquito-borne diseases. Climate change is the long-term change in the average weather conditions for a particular location. Climate change will become apparent as a change in annual, seasonal, or monthly means. Thus, incremental climate change will be superimposed upon the natural variability of climate in time and space.

Natural Disasters. Climate change will increase the risk of both floods and droughts. Ninety percent of disaster victims worldwide live in developing countries, where poverty and population pressures force growing numbers of people to live in harm's way—on flood plains and on unstable hillsides. Unsafe buildings compound the risks. The vulnerability of those living in risk-prone areas is perhaps the single most important cause of disaster casualties and damage.

Water Quality and Quantity. Human health depends on an adequate supply of potable water. By reducing fresh water supplies, climate change may affect sanitation and lower the efficiency of local sewer systems, leading to increased concentrations of pathogens in raw water supplies. Climate change may also reduce the water available for drinking and washing. In developed countries, the anticipated increase in extreme rainfall events, which may be associated with the outbreaks of diarrheal diseases, may overwhelm the public water supply system. Flooding is likely to become more frequent with climate change and can affect health through the spread of disease. In vulnerable regions, the concentration of risks with both food and water insecurity can make the impact of even minor weather extremes (floods, droughts) severe for the households affected. The only way to reduce vulnerability is to build the infrastructure to remove solid waste and waste water and supply potable water. No sanitation technology is "safe" when covered by flood waters, as fecal matter mixes with flood waters and is spread wherever the flood waters go.

Food Security. Current assessments of the impact of climate change indicate that some regions are likely to benefit from increased agricultural productivity while others may suffer reductions, according to their location and dependence on the agricultural sector. The IPCC has reviewed the results of many modeling experiments that project future changes in crop yields under climate change. Climate change may increase yields of cereal grains at high and midlatitudes but may decrease yields at lower latitudes. The world's food system may be able to accommodate such regional variations at the global level, with production levels, prices, and the risk of hunger being relatively unaffected by the additional stress of climate change. However, populations in isolated areas with poor access to markets may still be vulnerable to locally important decreases or disruptions in food supply.

Heat Waves and Milder Winters. Heat stress is a direct result of exposure to high temperatures. Stressful hot weather episodes (heat waves) cause deaths in the elderly, as well as heat related illnesses such as heat stroke and heat exhaustion. A change in world climate, including an increase in the frequency and severity of heat waves, would affect the quality of life in many urban centers. Heat waves are responsible for a significant proportion of disease-related mortality in developed counties such as the United States and Australia, where the impact of weather disasters has been significantly reduced. Milder winters under climate change would reduce the excess morbidity and mortality, such as the United Kingdom, the beneficial impact may outweigh the detrimental.

Air Pollution. The air is full of particles and gases that may affect human health, such as pollen, fungal spores, and pollutants from fossil fuel emissions. Weather conditions influence air pollution via pollutant (or pollutant precursor) transport and/or formation. Exposures to air pollutants have serious public health consequences. Climate change, by changing pollen production, may affect timing and duration of seasonal allergies.

Social Dislocation. The growth in the number of refugees and displaced persons has increased markedly. Refugees represent a very vulnerable population with significant health problems. Large-scale migration is likely in response to flooding, drought, and other natural disasters. Both the local ecological disturbance caused by the extreme event and the circumstances of population displacement and resettlement would affect the risk of infectious disease outbreaks. Even displacement due to long-term cumulative environmental deterioration, including sea level rise, is associated with such health impacts.

Infectious Diseases. Vector-borne diseases are transmitted by insects (e.g., mosquitoes) and ticks that are sensitive to temperature, humidity, and rainfall. Climate change may alter the distribution of important vector species, and this may increase the risk of introducing disease into new areas. Temperature can also influence the reproduction and survival of the infective agent within the vector, thereby further influencing disease transmission in areas where the vector is already present. However, the ecology and transmission dynamics of vector-borne diseases are complex. The climate factors that could critically influence transmission need to be identified before the potential impact of a changing climate can be assessed.

Malaria is on the increase in the world at large, but particularly in Africa. In several locations around the world, malaria is reported in the twenty-first century at higher altitudes than in preceding decades, such as on the mountain plateaus in Kenya. The reason for such increases has not yet been confirmed but include population movement and the breakdown in control measures. Climate change may contribute to the spread of this major disease in the future in highlands and other vulnerable areas. Climate change impact models suggest that the largest changes in the potential for disease transmission will occur at the fringes—in terms of both latitude and altitude—of the potential malaria risk areas. The season transmission and distribution of many diseases that are transmitted by mosquitoes (dengue, yellow fever), sandflies (leishmaniasis), and ticks (Lyme disease, tick-borne encephalitis) may also be increased or decreased by climate change.

ADAPTATION AND MITIGATION

There are two responses to global climate change:

• Mitigation. Intervention or policies to reduce the emissions or enhance the sinks of greenhouse gases. The current international legal mechanism for countries to reduce their emissions is the United Nations Framework Convention on Climate Change (UNFCCC).
• Adaption. Responses to the changing climate (e.g., acclimatization in humans) and policies to minimize the predicted impacts of climate change (e.g., building better coastal defenses).

The key determinants of health—as well as the solutions—lie primarily outside the direct control of the health sector. They are rooted in areas such as sanitation and water supply, education, agriculture, trade, transport, development and housing. Unless these issues are addressed, it can be difficult to make improvements in population health and reduce vulnerability to the health impacts of climate change.

R. Sari Kovats

(see also: Environmental Determinants of Health; Geography of Disease )

Bibliography

Houghton, J. T.; Meira Filha, L. G.; Callander, B. A.; Harris, N.; Kattenberg, A.; and Maskell, K., eds.(1996). "The Science of Climate Change." Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.

McMichael, A. J., and Haines, A. (1997). "Global Climate Change: The Potential Effects on Health." British Medical Journal 315:805–809.

Patz, J. A.; McGeehin, M. A.; Bernard, S. M.; Ebi, K. L.; Epstein, P. R.; Grambsch, A.; Gubler, D. J.; and Reiter, P. (2000). "The Potential Health Impacts of Climate Variability and Change for the United States: Executive Summary of the Report of the Health Sector of the United States National Assessment." Environmental Health Perspectives 108:367–376.

Watson, R.; Zinyowera, M. C.; Moss, R. H.; and Dokken, D., eds. (1996). "Climate Change 1995. Impacts, Adaptations, and Mitigation of Climate Change: Scientific and Technical Analyses." Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.

climate change and lake levels Lakes occupy a mere 1 per cent of the Earth's land area, but they are of great importance in reconstructing its past environments. Just as mountains are areas of net erosion and sediment loss, so lakes are subject to net accumulation and sediment gain. The resulting lacustrine sediments preserve a record of past ecological conditions and geomorphic processes, which can be studied by techniques such as pollen analysis and magnetic measurements. Indeed, the build-up of sediment means that most lakes are relatively ephemeral features, as they eventually fill up and become terra firma.

As well as acting as a natural ‘archive’ for the surrounding landscape, lakes can also provide a valuable record of regional climate change via study of fluctuations in their water level and salinity. While all lakes respond to changes in water balance, the most responsive and easily quantified are those which are hydrologically closed. These ‘non-outlet’ lakes adjust their surface area and water depth dynamically in response to changing inputs in the form of rainfall and river flow, and to changing losses in the form of evaporation. Among the most important non-outlet lake basins are the Caspian Sea, Lake Eyre in Australia, and the Great Salt Lake in North America. Climatically induced fluctuations in these and many other smaller lakes have been documented in recent decades through historical records and remotely sensed imagery. Lake Chad, for example, has shrunk dramatically in area during the late twentieth century. This shrinkage has been associated with the Sahel droughts of the 1970s and 1980s (Fig. 1a). Similarly (although for different reasons), the surface area of the Aral Sea has shrunk by 40 per cent since 1960, with disastrous ecological and human consequences.

Lake hydrology

The water balance of any lake is determined by the inputs, namely precipitation onto the lake surface, inflowing streams, surface run-off and subsurface inflow, e.g. springs; and the outputs, namely evaporation from the lake surface, outflowing streams or rivers, and subsurface outflow, e.g. sink holes. If a lake is in hydrological equilibrium, inputs and outputs will balance. In a hypothetical case where groundwater exchanges are negligible, then the variable which adjusts to maintain equilibrium is surface outflow. As a lake water surplus increases, so the outflow discharge becomes greater to compensate for it. It is possible, however, that there will be no water surplus from the lake, in which case there will be no surface discharge and the lake will be hydrologically closed.

In a lake without an outlet, hydrological equilibrium is maintained, not by changes in outflow discharge, but through adjustment of the area of the lake and hence by net evaporation loss. Although the surface area is the important hydrological parameter for a lake, it will co-vary with lake water level. The former can be calculated from the latter by means of the area-depth hypsometric curve determined from the individual morphometry of each lake. It is this fundamental hydrological relationship which links climate to fluctuations in lake level, and enables the past water balance and climate to be reconstructed from palaeolimnological data (i.e. data from ancient lakes).

In reality, few lakes fulfil the assumptions set out above in acting like giant ‘rain gauges’, particularly because of groundwater flows. A lake can be closed in terms of surface hydrology, but may not be isolated from subsurface inflows and outflows. Indeed some lakes, such as those in the Corangamite region of western Victoria in Australia, are largely fed by subsurface inflows and act essentially as groundwater ‘windows’.

Lake levels and salinity

When a lake moves from being open to closed, it usually also changes from being freshwater to saline. Instead of being washed away down the outflow stream, solutes (salts) are retained within the lake water and are progressively concentrated by evaporation. As lake levels fall, the remaining water initially becomes brackish, later fully saline, and eventually hyper-saline like the Dead Sea. For this reason most arid-zone lakes are salt lakes. If, on the other hand, the level of a lake rises to the point of overflowing, salts are then flushed out of the system and the lake water becomes fresh. Past salinities are recorded in the chemical and isotopic composition of lake sediments. In a lake with an outflow, sediments such as freshwater diatomite or gyttja (organic mud) will be deposited, but if a lake moves from positive to negative water balance and the water becomes chemically concentrated, carbonates of various types will be laid down. The subsequent chemical evolution normally follows one of two main pathways, depending on the initial composition of the water. One pathway leads towards carbonate being the dominant anion and the eventual precipitation of salts such as trona (a form of hydrous sodium carbonate); the other leads towards chloride or sulphate dominance, or both, and the formation of compounds such as gypsum. The presence of these kinds of evaporite in a sedimentary sequence would be indicative of a hyper-saline, ‘playa’-type lake environment. Evaporative concentration also influences the stable isotope ratio of the sediments, and, in particular, the oxygen-isotope ratio has proved valuable in palaeolimnological reconstruction.

This alternation between fresh and saline conditions not only influences the character of the sediments deposited in the lake, but also largely controls the organisms living within it. When preserved after death in the lake-bottom sediments, their remains can be used to reconstruct past salinities and water depths, and hence also climate. Among the most useful of these indicator organisms are diatoms, ostracods, and molluscs. Figures 1(b) and 1(c) show a historically reconstructed water-level curve from Lake Naivasha in Kenya, together with the abundance of head capsules of a salt-tolerant chironomid (midge larva) from a short sediment core taken in Oloidien Bay. This arm of the main lake is isolated and saline during periods of low water level, as occurred before 1887 and again in the middle decades of the twentieth century, and at these times this chironomid species was relatively abundant. Much recent research on biological indicators has gone on to use statistical methods to reconstruct past lake salinity, especially from diatoms.

Where the regional bedrock is highly permeable, salts may not be retained in the lake, and in this case water-level fluctuations will not be accompanied by significant changes in lake salinity. This is true, for example, of the groundwater-fed lakes of the Parkers Prairie Sand Plain in Minnesota and those of the limestone massifs in Morocco's Middle Atlas mountains. At these sites, fluctuations in water depth rather than in salinity provide the main record of past changes in water balance and climate. Former high lake levels are also indicated by geomorphological evidence, notably as shoreline terraces left above the present-day water surface.

Lake-based climate reconstruction

Various methods have been used to calculate past precipitation levels for individual lake basins. Two of the principal methods are the simple water balance and combined water- and energy-balance approaches. In the first of these, the former lake area is used to calculate total evaporation losses from the water surface, which can then be balanced against the input of precipitation plus runoff from the catchment. The chief assumption that this approach makes is that past temperatures—the primary control over evaporation rates—are known. In the second approach, the evaporation term is eliminated between the water- and energy-balance equations, but it is necessary to estimate certain surface properties such as albedo and the Bowen ratio (a function of vapour pressure and temperature, which is used in the assessment of evaporation). These properties are related to the former vegetation. Both these and area-based approaches have been used to provide estimates of past precipitation in regions such as inter-tropical Africa. They indicate an average annual increase in rainfall of at least 250 mm for the Sahara, East Africa, and South Asia about 9000 (radiocarbon) years ago.

It was originally believed that the pluvial (wet) conditions responsible for climatic changes in lower latitudes matched up with the glacial (or cold) periods of high latitudes—and in a few subtropical areas, notably the American south-west, this appears to have been the case (see pluvial lakes). However, for many other regions, particularly those within the tropics, radiocarbon dating has now shown that the last wet period occurred during the Late Pleistocene and the first half of the Holocene (between 12 500 and 5000 years ago). In areas such as the Sahel, lakes which today are saline or dry contained fresh water during the early Holocene, and were able to support rich and varied aquatic faunas. The increased wetness and higher lake levels of the early Holocene appear to have been brought about by an intensification and northward displacement of the Indian Ocean monsoonal circulation, itself related to orbital forcing associated with Milankovich cycles.

Because of their typically rapid response time, lake levels also provide important palaeoclimatic data on shorter-term ‘Sub-Milankovich’ timescales. Many of the more detailed lake-level records show significant fluctuations lasting between 100 and 1000 years. These mark abrupt climatic events produced by phases of major explosive volcanism, reorganization of oceanic circulation, or similar factors. At least one of these events, the Younger Dryas, appears to have been global in nature, and is marked in the tropics by a sharp fall in lake levels, indicating a period of climatic aridity. Lakes—or at least their remains in the form of lacustrine deposits—therefore provide valuable evidence for the changing climate and hydrology of the world's dry lands over a hierarchy of timescales during the late Quaternary.

Neil Roberts

Bibliography

Almendinger, J. E. (1993) A groundwater model to explain past lake levels at Parkers Prairie, Minnesota, USA. The Holocene, 3, 105–15.
Street-Perrott, F. A. and and Harrison, S. P. (1985) Lake levels and climate reconstruction. In Hecht, A. D. (ed.) Paleoclimate analysis and modeling, pp. 291–340. John Wiley and Sons, New York.
Street-Perrott, F. A. and and Roberts, N. (1983) Fluctuations in closed-basin lakes as an indicator of past atmospheric circulation patterns. In Street-Perrott, A., Beran, M., and Ratcliffe, R. A. S. (eds) Conference on variations in the global water budget, pp. 331–45. Reidel, Dordrecht.

Global Climate Change

Energy from the Sun passes through the atmosphere as light and is absorbed by soil, rock, and water at the surface of Earth. The energy is reradiated as heat and absorbed in the atmosphere by greenhouse gases, including carbon dioxide (CO₂), water vapor, methane, ozone, nitrous oxide, and the human-made chemicals chlorofluorocarbons (CFCs). This atmospheric warming is called the greenhouse effect; without it Earth's average global temperature would be about –18 degrees Celsius (0 degrees Fahrenheit). Greenhouse gases are added to the atmosphere by natural events including volcanic eruptions, the decay and burning of organic matter, and respiration by animals. They are also removed from the atmosphere. CO₂ is absorbed by seawater and stored in plant tissue. When plants die and gradually are transformed into fossil fuels—coal, oil, natural gas—deep in the earth, their CO₂ is stored with them. The removal of greenhouse gases from the atmosphere keeps the planet from overheating.

Climate History

Besides the concentrations of greenhouse gases in the atmosphere, other factors affect global climate including Earth's orbital behavior, the positions and topography of the continents, the temperature structure of the oceans, and the amount and types of life. During much of Earth's history the climate was warm and humid with ice-free poles; global average temperatures were about 5 degrees Celsius (9 degrees Fahrenheit) higher than today. Several times glaciers covered the higher latitudes, most recently during the Pleistocene (1.6 million to 10,000 years ago), when up to 30 percent of the land was covered by ice. During the four glacial advances of the Pleistocene, average global temperature was 5 degrees Celsius lower than today and 10 degrees Celsius (18 degrees Fahrenheit) lower than the ancient global average. During the three interglacial periods, global temperature was a degree or two warmer than today. Many scientists think that Earth is in an interglacial period, and the ice sheets will return.

Since the peak of the last glacial advance 18,000 years ago, average global temperature has risen 4 degrees Celsius (7 degrees Fahrenheit), including 1 degree Celsius (1.8 degrees Fahrenheit) since the beginning of the Industrial Revolution. It is difficult to know how much of the recent warming is the result of the end of the Pleistocene and how much is the result of human activities that add greenhouse gases to the atmosphere. CO₂ is the most abundant greenhouse gas, a by-product of burning fossil fuels and modern forests. In the early twenty-first century, there is greater than 30 percent more CO₂ in the atmosphere than in 1850. There have also been significant increases in methane and CFCs. Some projections show a doubling of CO₂ over preindustrial levels by 2050 and additional increases in methane. (CFCs are being phased out by international agreement because they destroy Earth's protective ozone layer.)

Adding greenhouse gases to the atmosphere is like throwing another blanket on Earth; the consequent rise in global temperature is known as global warming. Since climate is a complex system and climate models are difficult to construct, scientists can only speculate on the effect large increases in greenhouse gases will have on global climate. Some models show average global temperature increasing as much as 5 degrees Celsius by 2100. Any temperature increase will not be uniform. Since ocean water absorbs more heat than land, the Southern Hemisphere (which has more water) will warm less than the Northern. Atmospheric circulation patterns will bring the greatest warming, as much as 8 to 10 degrees Celsius (14 to 18 degrees Fahrenheit), to the poles.

Possible Consequences

A rapid increase in global average temperature could have profound effects on social and natural systems. Warmer temperatures would cause ocean water to expand and polar ice caps to melt, increasing sea level by as much as 50 centimeters (1.6 feet) by 2100. This would flood coastal regions, where about one-third of the world's population lives and where an enormous amount of economic infrastructure is concentrated. It would destroy coral reefs, accelerate coastal erosion, and increase salinity to coastal groundwater aquifers. Warmer temperatures would allow tropical and subtropical insects to expand their ranges, bringing tropical diseases such as malaria, encephalitis, yellow fever, and dengue fever to larger human populations. There would be an increase in heat-related diseases and deaths. Agricultural regions might become too dry to support crops, and food production all over the world would be forced to move north; this would result in a loss of current cropland of 10 to 50 percent and a decline in the global yield of key food crops of from 10 to 70 percent.

Wild plant and animal species would need to move poleward 100 to 150 square kilometers (60 to 90 miles) or upward 150 meters (500 feet) for each 1 degree Celsius rise in global temperature. Since most species could not migrate that rapidly and since development would stop them from colonizing many new areas, much biodiversity would be lost. The decrease in the temperature difference between the poles and the equator would alter global wind patterns and storm tracks. Regions with marginal rainfall levels could experience drought, making them uninhabitable. Overall, since warmer air holds more moisture, an increase in global air and sea temperatures would increase the numbers of storms. Higher sea surface temperatures would increase the frequency and duration of hurricanes and El Niño events.

Many scientists believe that global warming is the most serious threat to our planet. By 2025 the world's energy demand is projected to be 3.5 times greater than in 1990, with annual CO₂ emissions nearly 50 percent higher. Thus far, attempts at international agreements to curb the emissions of greenhouse gases (for example, the Kyoto Protocol) have failed. This is due to several factors: (1) the scientific uncertainty of the role humans play in global warming; (2) the lifestyle changes necessary to reduce fossil fuel consumption in developed nations; (3) the possible slowdown in the economic development of developing nations; and (4) the need for true international cooperation. A high-technology alternative to decreasing greenhouse gas emissions is to sequester CO₂. Experiments are underway to inject liquid CO₂ deep into the earth, thereby effectively removing it from Earth's carbon cycle.

see also Biogeochemical Cycles; Carbon Cycle; Ecological Research, Long-Term; Ecosystem; Extinction; Tundra

Dana Desonie

Bibliography

Drake, Frances. Global Warming: The Science of Climate Change. Edward Arnold, 2000.

Stevens, William K. The Change in the Weather: People, Weather, and the Science of Climate. Delta, 2001.

climate change. The earth's climate continually fluctuates as a result of the earth's elliptical orbit around the sun. When the earth is closer to the sun it receives more radiant energy. Long-term analyses of climate based on deep ocean deposits and ice cores from the Antarctic and Greenland ice caps show there are climatic cycles lasting 25,000, 40,000, and 100,000 years, which have resulted in the oscillations between glacial and interglacial periods. There are also millennial scale oscillations: the Vikings were able to colonize Greenland in their longboats during a mild climatic period about a thousand years ago, and from there visited Vinland, whereas there was a mini-ice age in the early 18th century when goose fairs were regularly held on the frozen Thames. Recently, 70-year cycles have been identified in the North Atlantic Oscillation. When pressure is high over the Azores, and low over Iceland, the flow of the Gulf Stream increases, keeping north-western Europe's climate mild. In the opposite phase the flow reduces and the climate of Europe is cooler. Similar oscillations have been identified in the North Pacific and both may be linked with El Niño events.

The gases in past atmospheres can be measured in the gas bubbles trapped deep in the ice caps. These show that as climate has oscillated so have the amounts of carbon dioxide in the atmosphere. When CO₂ concentrations are high, more of the incoming radiant heat from the sun is trapped in the atmosphere—the so-called greenhouse effect. Other gases such as methane and CFCs (chlorofluorocarbons) are many more times more effective in trapping the heat, but their concentrations are rising more slowly. This link between carbon dioxide and climate is well established, but exactly what causes it is a matter of controversy. Since the start of the Industrial Revolution in the 18th century the burning of fossil fuels has increased the carbon dioxide content of the atmosphere from pre-industrial levels of 240 parts per million (ppm) to present levels of over 370 ppm. If we continue to burn oil and coal at the present rates it will rise to 500–600 ppm by the end of the century. As more and more of the sun's energy gets trapped in the atmosphere, the earth's climate will warm, but not uniformly across the planet; polar regions will warm more than the tropics. Although some countries will benefit from such shifts most will lose out. Sea levels will rise as the polar ice caps melt and in 2003 Australian scientists produced fresh evidence showing that the area of sea ice surrounding Antarctica has shrunk by 20% in the last 50 years. Weather patterns will become less predictable with bigger and more destructive storms. Shifts in rainfall will increase flooding in some places, but cause droughts and desertification in others. The pattern of ocean currents will shift, and close links between ocean currents and local climate are contributing to the uncertainties of predicting the outcome of global climate change and are discouraging governments from taking the social and economic sacrifices needed to halt the rise in carbon dioxide emissions.

Already ocean plankton off the west coast of Britain has become more typical of warmer seas, and there have been sharp decline in the abundances of keystone species, such as the crustacean copepod (Calanus finmarchicus), which has been linked to the catastrophic decline in cod stocks. Climate will continue to fluctuate because many of the causes are planetary, but the implications of the changes will become increasingly threatening as world population grows and industralization increases globally, one of the most serious environmental issues facing us today.

M. V. Angel

climate change and laminated sediments Laminated sediments can often be used to infer short-term (e.g. seasonal to annual) climatic or oceanographic fluctuations, since each lamina is comprised of sediment deposited under unique conditions. For example, annual winter rains lead to the deposition of silty laminae in the Santa Barbara Basin, off the coast of California. A first step in deciphering past climate records from laminated sediments is thus to identify the sedimentary components of successive laminae. This can be done in situ by using data from sediment traps. Sediment traps continuously collect descending sediment (e.g. biogenic or terrigenous material), collection periods sometimes being for a year or more. Over such periods of time, any seasonal changes in sedimentation patterns will be observed, such as increasing silt deposition due to increased rainfall or glacial discharge, or abundant diatoms due to algal blooms. Such observations can be compared with the laminated sedimentary record observed directly from the sediments. Advances in scanning electron microscope (SEM) back-scattered electron imaging techniques make it possible to examine micron-scale lamina variations in rock sections; these can provide detailed records of sub-annual deposition. Data from both sediment-trap studies and SEM examination of core intervals can be compared with his-torical climate records. The interpretations provided by such studies greatly aid both the study of laminated sediments that predate historical records and the understanding of longer-term variations in palaeoclimate and palaeoceanography.

Many studies of hand or microscopic specimens analyse specific features of laminated sedimentary sections; for example, colour or grey-scale changes. The occurrence through time of any variations can be documented, and their statistical significance investigated. The periodicity of any significant frequencies is in some instances related to climatic or oceanographic phenomena; for example, periodicities between 2 and 7 years correspond to the periodicity of the El Niño-Southern Oscillation (ENSO).

Laminated sediments occur in various depositional environments, but primarily where water is poorly oxygenated at the sea or lake bed, that is, where water is either dysaerobic (oxygen-poor) or anoxic (no oxygen present at all). Under these conditions, the activity of benthonic organisms is severely or entirely curtailed, and individual laminae are preserved. Typical locations where this occurs are: (1) basins with limited deep water replacement; for example, Californian Borderland Basins; (2) areas of intense upwelling, which experience high organic-carbon sedimentation rates owing to increased primary productivity, for example, the Gulf of California, and off the coast of Peru; and (3) lakes, where stratification of the water column often prevents oxygenation of the deeper waters; for example, Arctic lakes and high alpine lakes. More recently, deep-sea laminated diatom oozes have been recovered from the Pacific and Atlantic Oceans, making it possible to investigate short time-scale oceanographical processes such as Miocene- Pliocene El Niño-like events in the eastern equatorial Pacific. In the ancient sedimentary record, laminated sediments are preserved in marine sediments from periods of ocean anoxia, and similarly from freshwater and saline lakes, providing insights into ancient climate and ocean change.

R. B. Pearce

Bibliography

Kemp, A. E. S. (ed.) (1996) Palaeoclimatology and palaeoceanography from laminated sediments. Geological Society Special Publication No. 116.

climate records

cli·mate change • n. long-term, significant change in the climate of an area or of the earth, usually seen as resulting from human activity. Often used as a synonym for global warming.