Geology 108. Crises of the Earth

Global Climate Change Module

©Bill Leeman


Animation of sea surface-temperature variations associated with the El Nino phenomenon (NOAA)


The fate of the Maldives: signs of global warming?

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Climate History

The Earth (Blue Planet) is unique in our solar system in having abundant water at its surface. The geologic record provides clear evidence that the Earth's climate has varied significantly with time. Life on Earth has evolved largely as a consequence of changing conditions (esp. atmospheric composition) with time. The evidence comes from stratigraphic variations in geologic deposits (esp. sediments), and the chemistry of these deposits or fossils within them. (A more detailed geologic time scale.)

 

Record of Climate Change

Climate vs. weather

History of climate change

 

Causes of Climate Variations

Periodicity of global temperature has been verified by several means (direct and indirect). Statistical analysis of paleo-temperature records (time-temperature 'spectra') reveals that three major astronomical forcing factors influence the amount of solar energy received by the Earth and the distribution thereof. These factors are related to eccentricity of Earth's orbit, tilt of the planet rotation axis, and precession of this axis - all of which vary significantly with periodicities of ~100 ka yrs, 41 ka, and 19-23 ka, respectively.

Catastrophic events such as meteorite/comet impacts or volcanism can also influence climate variations on the short-term by perturbing the normal atmospheric conditions, and affecting the transfer of solar energy. (cf. effects of the 1991 Pinatubo eruption)

Geologic factors also affect climate.

Biologic activities have undoubtedly influenced the composition of Earth's atmosphere over time.

Human activities (esp. since the industrial revolution) also have dramatic impacts on climate.

 

Problems related to global warming

Future warming scenarios

Rio Conference on Global Climate Change

Melting of polar ice sheets and sea level rise


The Challenge - Modelling the Future (adapted from DKRZ)

Humanity has arrived at a historical crossroad. For millenia, nature was considered inviolable and the earth's resources inexhaustible. In the past few decades, however, the world population has been growing rapidly, and each individual is using more and more resources. We have been forced to recognize that our planet's capacity is limited and that nature's balance is vulnerable. Today, humans interfere with nature's regulative processes to an unprecedented degree, causing uncontrolled long-term changes in living conditions on earth. The global environment, and the climate in particular, are extremely complex systems, whose internal dynamics and future evolution can only be understood through extensive investigations and complicated model computations. An understanding of these natural processes and the implications of man's interference is, however, a prerequisite for preserving the diversity of living conditions on earth. The public and policymakers therefore expect science to provide a reliable basis for climate and environmental protection policy. The earth cannot be experimented with. It is too complex to be reproduced in the laboratory. The only remaining possibility is to simulate it in a computer model.

How will the climate system react in the future to diverse emissions of greenhouse gases, nitrogen oxides, and other trace substances? How will global warming, the increase in sea level and the change in composition of the upper and lower atmosphere affect living conditions for humans? In order to analyse these impacts, researchers must consider both changes resulting from human activity and those resulting from natural climate and environmental fluctuations, which have historically exerted a heavy stress on societies. The assessment of how the climate in the next century will be affected by the combination of both natural and human factors represents one of the major challenges for science.

 

Climate Change

Public interest has focussed mainly on climate change through human activity. However, the problem of anthropogenic climate change cannot be separated from the question of natural climate variability, which has occurred in all epochs of the earth's history. Only when climate research has succeeded in explaining natural climate variability will it be possible to assess the impact of future climate change caused by humans. It is generally believed that the most damaging effects of an anthropogenic change in climate will be experienced foremost in the changed frequency of extreme events (storms, droughts, floods, winters with little snow, etc.). An understanding of natural climate variability is also a prerequisite for the early detection of anthropogenic climate change: anthropogenic climate modifications can clearly be distinguished from natural climate fluctuations only if the structure of the natural fluctuations are known.

In the past, a strong focus of the research on natural climate variability has been on the causes and predictability of short-term climate changes. A striking example for this is the ENSO phenomenon (El Niño/Southern Oscillation). El Niño, "the Christ Child", is the name given by the fishermen on the Peruvian Pacific Coast to the warm ocean current which occurs every three to five years around Christmas. It causes a collapse of the Pacific trade wind system during that year, producing repercussions throughout the global climate system. El Niño leads to harvest failures in Australia, floods in California and even affects the Indian monsoon. Using coupled ocean-atmosphere models, it has now become possible to simulate the complex interactions between the ocean and the atmosphere which modify the ocean currents and to predict the development of an El Niño up to 18 months in advance. Predictions of this type are important not only for tropical regions, but for the entire global economy.

 

The Ocean, the Flywheel of Climate

The significance of the ocean for climate has long been recognized. Through its large inertia it acts like a giant flywheel, delaying and dampening the effects of external influences on the climate system. With a density approximately a thousand times higher than that of air, the ocean can absorb far more heat than the atmosphere. The three upper metres of the ocean alone contain as much heat as the entire atmosphere above it. The ocean absorbs and releases heat only slowly. It takes over two months, for example, for the uppermost 50 m layer of the ocean, which is well mixed by the wind and waves, to adjust to the changing insolation of the sun during the seasonal cycle. Since the surface temperature of the ocean has a strong influence on air temperature, this results in the well known lag of the summer and winter seasonal extremes by several weeks behind the maximum and minimum position of the sun. For the considerably longer periods of decades to millenia which are relevant for climate change, the significantly larger heat capacity of the deep ocean must be taken into account.

Deep sea currents and the surface layers of the ocean are coupled by the sinking of surface water at high latitudes, mainly in the North Atlantic and in the Antarctic. Cooling increases the density of the surface water until it becomes heavier than the water beneath, when it begins to sink. Strong cooling in the winter months can result in sinking all the way down to the ocean bottom. The vertically mixed water masses are then transported horizontally by the wind-driven ocean current systems. The heat transport generated in this manner tends to equalize the global distribution of temperatures, reducing, in particular, the contrast between the warm equatorial and cold polar regions. Together with the heat transport by the atmospheric wind systems, which is of the same order of magnitude, the ocean circulation thus represents an important regulatory mechanism for maintaining a stable climate on earth. Since the ocean reacts to changed conditions much more slowly than the atmosphere, it can imprint its history on atmospheric climate. Still today, the deep waters of the oceans carry remnants of the climate changes following the "little ice age" from the 15th to 17th centuries. Anthropogenic modifications of the deep water formation in the North Atlantic could have a similar or even greater effect on our future climate.

 

The Role of the Gulf Stream

The North Atlantic is a particularly sensitive area of the global climate system. Every second, approximately 17 million cubic metres of water sink to the depths of the North Atlantic - about 20 times the water flowing through all the rivers on earth. In the deep ocean the water then drifts southwards at a sluggish pace of a few kilometres per month. The deep current is balance by a return flow of warm water northward from the Caribbean. This warm current, the Gulf Stream, is responsible for Europe's mild climate. Relaltively small changes in the atmospheric forcing can reduce the deep water formation in the North Atlantic sufficiently to cause a breakdown of the Gulf Stream and a drop in the temperatures in Europe by several degrees.

 

The Ocean as Carbon Reservoir

The ocean stores and transports not only heat but also carbon dioxide (CO2). Only about half of the carbon dioxide released by humans through use of fossil fuels and the destruction of forests is still in the atmosphere today. The ocean is considered to be the major sink for the remaining half. In the long term, about 85 per cent of the CO2 emitted into the atmosphere will eventually be dissolved in the ocean. However, this process takes several hundred years. As long as the anthropogenic CO2 emissions keep increasing as in recent years, the high potential storage capacity of the ocean cannot become effective. Phytoplankton in the ocean also "consumes" carbon dioxide from the atmosphere. The major portion of this uptake is released via the food web back into the atmosphere, but a small amount is deposited onto the ocean floor in the calcium carbonate shells of dead organisms, thereby finally removing CO2 from the atmosphere-ocean system - after a few thousand years.

 

Scenario computations

Climate models have evolved today to a level where realistic simulations of of the evolution of the future global climate can be carried out for given scenarios of future greenhouse gas emissions. Previous assessments of global warming were based on atmospheric models alone, including only a rudimentary mixed-layer ocean. Without a realistic representation of the "flywheel" ocean, they could be used only to compute the equilibrium climate change resulting, for example, from a doubling of CO2, not the time evolution and the delay in warming caused by the uptake and redistribution of heat by the ocean.

The development of the climate in the next century was computed for several specified scenarios of future greenhouse gas concentrations. One of these scenarios assumes that draconic measures are undertaken to reduce emissions of greenhouse gases, so that the atmospheric concentrations increase only slowly. In this case, the mean temperature increase by the year 2085 remains under 1 degree C. If, on the other hand, the atmospheric greenhouse gas concentrations continue to increase at the present rate, a mean global warming of 2-3 degree C is predicted within the next century. This would lead to substantial shifts in the climate zones of the earth, with unforeseeable consequences for the global economy and human living conditions.

The computed temperature changes for the end of the 21st century agree reasonably well with previous predictions by the IPCC (Intergovernmental Panel on Climate Change). However, the warming for the first fifty years differs from the IPCC values, the warming according to the DKRZ computations increasing rather more slowly during the next decades.

Of more interest than the global mean temperatures is the information on the regional distribution of climate change. Which regions will experience the largest warming? In particular, what is the regional impact of changes in the heat transport of the ocean currents? These questions can be answered reliably only with coupled ocean-atmosphere models which are able to represent the ocean circulation and its interaction with the atmosphere realistically.

Firstly, they clearly demonstrate the effect of the delayed warming caused by the heat uptake of the ocean. Thus the temperatures increase more slowly in the southern hemisphere, which is dominated by the ocean, than had previously been assumed. Surprisingly, in some regions an initial cooling occured, particularly in the North Atlantic and in the Antarctic Weddell Sea, the principal regions of deep water formation. This was caused by a change in the ocean circulation. In the North Atlantic, the circulation decreased by about 20%, since the precipitation - and thus runoff - increased, thereby reducing the salinity and density of the surface water which drives the deep circulation.

Further computer simulations at the DKRZ confirm that salinity is a critical factor determining the strength of the North Atlantic current system. For example, in a simulation of the Younger Dryas event, a sudden return of ice age conditions about 12,000 years ago towards the end of the last ice age, the North Atlantic heat pump was shut off within a few decades by an influx of fresh water from melting glaciers. The resulting decrease in salinity by only two parts per thousand reduced the density of the surface water in the North Atlantic sufficiently to offset the density increase by cooling. The surface water then no longer sank to the ocean bottom, causing a breakdown of the deep circulation and the Gulf Stream.

 

Ice Age Climate

The ice age climate had a stronger effect on the habitats in the high northern and southern latitudes than in equatorial zones. Shown here are the differences between the annual mean near-ground air temperatures for present-day climate and for the climate during the last ice age approximately 18,000 years ago, computed with the Hamburg atmosphere-climate model. Inputs for this computation included the surface temperatures for the world ocean, reconstructed from ocean cores, the carbon dioxide content of the air, derived from ice core data, and the slightly modified solar radiation at that time.The mean temperatures during the last ice age were about 4 degree C lower than today.

 

Formation of Northern Ice Sheets

Glaciers and ice sheets have a strong influence on climate, since they reflect incoming solar radiation more effectively than normal land and ocean surfaces. The growth or decay of large ice sheets occurs over periods of tens of thousands of years. Shown here is the distribution of ice on the land masses of the Arctic, computed with the Hamburg-Bremen ice sheet model.

 

Antarctic Circumpolar Current

The ocean current circumflowing the Antarctic - seen here as an intricate interlaced pattern - is the most powerful current system on earth. It exerts a strong influence on climate. The figure represents a snapshot of the mass transport, which is approximately proportional to the temperature at the surface.

 

El Niño

At irregular intervals of several years, an anomalous warming of the surface of the Pacific is observed. At the same time, the prevailing easterly wind direction over the western Pacific is reversed - an El Niño event begins (cf. more detail, Figure). Fisheries off the west coast of South America collapse, and climate anomalies occur worldwide. The initiation and development of an El Niño is controlled by the internal dynamics of the coupled ocean-atmosphere system. With the Hamburg ocean-atmosphere model, the pattern and intensity of an El Niño can be computed reliably over one year in advance. The upper panel shows the observed sea surface temperature anomaly of an El Niño in October, 1982. The lower panel shows the temperatures computed nine months in advance, using observations from January, 1982. Similar simulations have been produced by other researchers (e.g., NOAA, animations).

 

Influence of Volcanoes on Climate

A strong volcanic eruption can influence the climate for several years. Although the eruptions themselves are not yet predictable, their effects are. In June, 1991, Pinatubo (Philippines) ejected over 20 million tons of sulfur dioxide into the stratosphere. The resulting stratospheric aerosols changed the short and long wave radiation budget of the atmosphere. The ensuing dynamic processes caused the stratosphere to cool off in the far north. The anomalies in the high and low pressure areas during the winter following the eruption were predicted using the Hamburg atmospheric climate model. The pattern of the changes was well predicted, but the magnitude was underestimated.

 

The Global Water Cycle

Water is essential for life on earth. In the hydrological cycle it is transported as vapour, liquid or ice. Water vapour, which rises over tropical seas and then condenses into clouds, contributes significantly to the heating of the atmosphere. The Hamburg model reproduces the global water cycle successfully, showing good agreement with the results of measurements.

 

Winter and Summer Precipitation over Land

The distribution of precipitation averaged over latitude in the winter months December - February (DJF) and the summer months June - August (JJA) show reasonable agreement between model and observations. The representation is broken in the land-free zones around 60 degree S.

 

Climate and Vegetation

Potential habitats for various plant communities can be computed from the prevailing temperatures and precipitation. Shown are the ten-year mean vegetation patterns computed with climate models of three different resolutions: 500 km(T21), 250 km (T42) and 100 km (T106). A comparison of the 10-year mean and one-year mean vegetation patterns for the T42 model reveals the magnitude of climate fluctuations from year to year. The HIRHAM panel shows the result of a one year simulation with a high resolution (50 km) regional model. The required computing times for comparable simulation regions and times increase as one passes through the model sequence T21, T42, T106 to HIRHAM by a factor of about 10 for each model resolution increase. Despite the use of high performance computers, global climate simulations over longer periods therefore still remain limited in 1994 to T42 resolution.

 

Climate Change and Vegetation

Plant communities evolve and exist stably in regions in which the combination of local climate parameters, in particular the temperature and precipitation, are just right for a particular community. When temperature and precipitation patterns shift as a result of climate change, the "potential" (i.e. equilibrium) vegetation no longer coincides with the actual vegetation. The potential distribution of plant communities resulting from a tripling of atmospheric CO2 was determined from the change in climate computed with the Hamburg atmospheric model, using a spatial resolution of 250 km (see also previous figure).

 

Atmospheric Water Vapour over the Ocean

Satellite measurements of microwave radiation (19.35 and 85.5 gigahertz, corresponding to wavelengths of 1.5 and 0.3 cm, respectively) provide estimates of the content of water vapour in the atmosphere. The rising air masses of the tropical belt (the "engine" of the atmospheric circulation), with it heat pole near New Guinea, and the monsoon in the Indian Ocean can be clearly recognized.

 

The Carbon Cycle

About one hundred billion tonnes of CO2 are exchanged between the atmosphere and the oceans annually. The ocean contains about sixty times as much carbon dioxide as the atmosphere. In the cold high latitudes, the ocean absorbs CO2 from the atmosphere. The CO2 is released again by the warm surface water at lower latitudes, mainly in the Pacific. In the net sum, the ocean is a sink for CO2 - as is the terrestrial biosphere. The release of CO2 due to deforestation by burning can be clearly recognized in tropical areas. Together, the oceans and terrestrial plants take up only about half of the CO2 produced by humans. The rest remains in the atmosphere. In the Hamburg carbon model, plant growth is enhanced by an increased CO2 content of the atmosphere. The biosphere thus binds more atmospheric CO2 on land. This so-called CO2 fertilisation effect presupposes, however, a sufficient supply of nutrients and water. If these are lacking, a further increase in atmospheric CO2 will not necessarily lead to an increased CO2 uptake by the terrestrial biomass.

 

Increasing CO2 Content of the Atlantic

Anthropogenic CO2, produced primarily by combustion of fossil fuels, first enters the ocean in the uppermost mixed layer. The surface water sinks to greater depths only near Greenland and Antarctica. With a circulation time of centuries to millenia, the deep ocean has not yet dispersed the past anthropogenic CO2 uptake. In the long term - in the course of many centuries - only about 15% of the additional CO2 remains in the atmosphere. For recent emissions, however, only the decadal time scale is relevant. On this time scale, the ocean has been able to absorb only about half of the past emissions. The figure shows a north-south section through the Atlantic (see small map inset at bottom left).

 

Global Change Research

It must be expected that the problems of climate research will continue to become more closely intertwined in the future with the general problems of Global Change. The problem of global warming serves as example. The simulations carried out at the DKRZ do not represent predictions in the strict sense, but only scenario computations: the climate change is computed under various assumptions about future emissions of greenhouse gases which are freely chosen and not further justified. The results represent only one link in the chain of assessments needed by policymakers for the development of an effective climate strategy. In addition, decisionmakers need information on the effect of climate change on natural ecosystems, agriculture, forest and energy management, trade, political relationships and - in general - on the wide range of socio-economic conditions which determine the quality of human life. Measures regulating emissions have economic and social impacts which, in turn, can indirectly influence climate. Decisionmakers must be able to assess also these feedbacks. Models can give insight into many - if not all - of these questions. It will therefore be an important future responsibility of the DKRZ to support climate researchers and scientists of other disciplines in the cooperative development of integrated assessment models to clarify and quantify the interrelationships between the many divers aspects of Global Change.

 

Climate Change Scenarios

Humans influence climate by emissions of greenhouse gases and - to a lesser degree - by changes in land use. In 1990, the Intergovernmental Panel on Climate Change (IPCC) studied scenarios of the future greenhouse gas content in the atmosphere. Scenario A described an unabated increase in emissions, while scenario D represented a policy of comprehensive emission reductions. The predicted climate changes under these assumptions were computed at the DKRZ, using for the first time a realistic ocean-atmosphere model. The protection of climate through the adoption of appropriate measures was shown to be feasible: comprehensive measures can reduce global warming to an acceptable level. On the other hand, an unabated increase in emissions leads to a significant global warming of 2 - 3 degree C. The coupled ocean-atmosphere model shows significant regional differences. Oceanic heat uptake causes a substantial delay in warming over the oceans compared to land. This is particularly evident in the Southern Hemisphere. In the global mean, the oceans delay warming by several decades.


Prepared: 15 March 1999