Oceanography in the 21st Century - An Online Textbook
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Modeling the Climate System

This Much We Know

  1. Greenhouse gases keep earth warm.
  2. Greenhouse gas concentration is increasing, mostly a as a result of human activity, including deforestation and burning of fossil fuel.
  3. Earth's climate is warming due to increasing greenhouse gas concentrations in the atmosphere.
  4. Earth's climate is the result of many interacting systems.

But, What About The Next 100 Years?

Ii's tough to make predictions, especially about the future.
Yogi Berra

Forecasts of climate change are mostly based on models of the physical climate system assuming that the present rate of increase will continue and that carbon dioxide concentrations in the atmosphere will increase to about 700 parts per million by 2100. The Intergovernmental Panel on Climate Change predicts:

  1. Average surface temperature of earth will warm by 2°– 6°C.
  2. Sea levels will rise by about 0.5 m.

Accuracy of Forecasts For The Next Century

Are these forecasts accurate? The answer depends on many factors.

Carbon Dioxide Concentrations
Will carbon dioxide concentrations in the atmosphere continue at the present rate? The best answer is: We don't know.

  1. The answer depends on economic, political, and geological factors.
  2. How fast will countries develop? This depends on economic activity in all countries, but economic forecasts are not possible, even one year into the future.
    1. The Intergovernmental Panel on Climate Change has proposed several possible scenarios. The most widely used is called Business as Usual.
    2. The Business as Usual scenario assumes developing countries will have constant, high levels of economic growth and developed countries will have constant, low levels of growth. This will result in African countries having higher gross domestic products than the united States in one hundred years. Is this realistic?

      "The dimensions of the problem can be illustrated by the case of South Africa. In 2000, this country's GDP per head, converted from nominal values using exchange rates, was only 12% of the US level. By 2050, the A1 marker scenario projects that the per capita income of South Africans on this basis will have reached more than four times the US level in 2000, and about twice the level that the US will have reached in 2050. And by 2100, this scenario projects that the per capita income of South Africans will be approaching twenty times the US level in 2000, and more than four times the US level at the end of the 21st century.

      "In the case of the B1 marker scenario (and other scenarios in the B1 family, one of which yields the lowest levels of emissions in the course of the century), the projected levels of average income in both countries in 2100 are somewhat lower than in the A1 marker scenario, but the level of affluence of South Africans exceeds that of Americans by an even wider margin than in the A1 projections. The total output of goods and services in South Africa in 2100, according to these downscaled A1 scenario projections, will be comparable to that of the entire world in 1990."
      From Ian Castles in IPCC Emissions Scenarios: The Case for a Review.

    3. The forecasts also assume no global disasters such as a world war or a global pandemic killing many people.
  3. How will political decisions influence the use of fossil fuels? Will governments provide incentives for use of alternate energy sources such as nuclear power plants, solar energy farms, or wind power?
  4. Are there enough reserves of low-cost, easily used fossil fuels to continue the increase? How much oil, gas, and coal remain to be found?
    1. Will oil become too expensive to use in cars or power plants?
    2. Will we substitute coal or natural gas for oil?
    3. Will we begin to use methane hydrates?
  5. Because of this uncertainty, projected concentrations in the year 2100 range from 400 to 1200 ppm (parts per million).

Climate Models
Are climate models sufficiently complex? The best answer is: We don't know.

  1. Thus they must include atmospheric, land, and ocean components.
  2. They should resolve the important variability in time and space.
  3. And, they must run for hundreds of years.
  4. But detailed, complex models cannot run for long times. They cannot include all important processes. Compromises are needed.
  5. Many important climate processes are not well known.
    1. We don't know enough about how clouds will influence reflected solar radiation (sunlight).
    2. We know little about aerosols (microscopic particles in the air).
      1. How do they influence absorption and reflection of sunlight?
      2. How many, and what type will be emitted by human activity
    3. We don't know if solar activity will change, and how the change will influence climate.
    4. We don't yet know or understand all the important feedbacks in the climate system. Here are just a few oceanic examples.
      1. If ocean surface waters warm, will primary productivity by phytoplankton change?
      2. Will winds change? If they do, will they transport more or less iron to the ocean as dust particles?
      3. If iron transport changes, will primary productivity change?
      4. If ocean surface water warms, the stability of the ocean changes. How will this influence ocean currents and the upwelling of nutrients? Will El Niño change?
      5. We don't know if the tropical Pacific will become cooler (more like La Niño conditions) or warmer (more like El Niño), yet the tropical Pacific strongly influences global weather patterns (Vecchi et al 2008).
      6. Will the change in stability influence the deep circulation?
      7. Will changes in the deep circulation lead to abrupt climate change?
      8. Will warmer water lead to the release of methane from methane hydrates? Methane is a potent greenhouse gas.

Here is the uncertainty in just the radiative forcing part of climate models:

radiative forcing of earths climate
Radiative forcing of earth's climate with uncertainty of each term. The influence of aerosols (microscopic particles in the air) and changes in solar output are not well known. Click on the image for a zoom.
From Intergovernmental Panel on Climate Change, Fourth Assessment Report 2007, Working Group 1 Technical Summary.

Past Performance
We can gain some insight into the possible accuracy of climate models by observing how well they predict past change.

  1. Present models cannot determine how the tropical Pacific will respond to global warming.
  2. Most coupled ocean-atmosphere-land models cannot reproduce the present climate. They must be adjusted to get the correct climate. The adjustments are called flux adjustments because the flux of heat and water between the ocean and the atmosphere is not known well so it must be adjusted.
  3. Climate models cannot predict changes in past climate of earth. For example, they cannot predict abrupt climate change that occurred many times in the past.
  4. Climate models have predicted the observed warming over the past 15 years.

Overall
It is perhaps fair to say that future predictions will not be accurate.

  1. Some future processes cannot be modeled. We cannot predict:
    1. Epidemics, war, and political actions, and economic activity, all of which influence emissions of greenhouse gases. IPCC reports just assume various possible ranges of future emissions, with emphasis on "business as usual."
    2. Volcanic activity.
    3. Future changes in solar activity.

  2. Most or all models used in geoscience have been wrong. Earth systems are more complex than we know or can model.
    See Useless Arithmetic: Why Environmental Scientists Can't Predict the Future by Orrin H. Pilkey, Linda Pilkey-jarvis (2007), Columbia University Press.

    But I have studied the climate models and I know what they can do. The models solve the equations of fluid dynamics, and they do a very good job of describing the fluid motions of the atmosphere and the oceans. They do a very poor job of describing the clouds, the dust, the chemistry and the biology of fields and farms and forests. They do not begin to describe the real world that we live in. The real world is muddy and messy and full of things that we do not yet understand. It is much easier for a scientist to sit in an air-conditioned building and run computer models, than to put on winter clothes and measure what is really happening outside in the swamps and the clouds. That is why the climate model experts end up believing their own models.
    Freeman Dyson Heretical Thoughts About Science and Society, 8 August 2007.

  3. We will never be able to test the predictions.
    1. We have only one earth and one future.
    2. The future is not the same as the past. The ability to predict changes that occurred in the past does not mean that we will be able to predict future changes even if we can ignore political, economic, solar, and volcanic activity.

Consequences of Global Warming Are Uncertain

  1. Some areas, mostly polar regions will be much warmer. This is already happening.
  2. We do not know how much temperature will rise in smaller areas and in populated areas.
  3. We do not yet know how climate change will influence the distribution of rainfall, tropical storms, and droughts.
  4. We have only imperfect understanding of how warming will change ecosystems.
  5. The consequences are not all bad. Overall, economists estimate the benefits will be about the same as the costs. For example:
    1. Canada, New England, and Russia benefit from warmer winters and a longer growing season.
    2. But, in other areas the use of air conditioning in summer will rise.

Decadal Predictions Are Probably Useful

If forecasts for the next century are very uncertain, perhaps we can produce better forecasts for shorter times. Looking carefully at the assumptions used for climate forecasts, and at the complexity of the forecast models, we may conclude that the forecasts for the next few decades may have useful accuracy.

  1. Is a third world war likely? Not really.
  2. Will there be a global pandemic that greatly slows global carbon emissions? Possibly, but not likely.
  3. Will economies continue to grow at the present rate? Maybe, but not in the next few years.
  4. Will fossil fuels become so expensive that they will be replaced by alternate energy sources? Possibly, it is already happening. But, fossil fuels will be the main source of power for the next few decades.
  5. Do we understand all feedback mechanisms in the earth system? No, but we probably understand the most important feedbacks.
  6. Are the models sufficiently complex? Probably.

    Advances have been made in the simulation of past climate variations. Independently of any attribution of those changes, the ability of climate models to provide a physically self-consistent explanation of observed climate variations on various time scales builds confidence that the models are capturing many key processes for the evolution of 21st-century climate. Recent advances include success in modelling observed changes in a wider range of climate variables over the 20th century (e.g., continental-scale surface temperatures and extremes, sea ice extent, ocean heat content trends and land precipitation).
    From IPCC (2008).

  7. Will earth climate continue to warm. Probably, because the excess carbon dioxide in the atmosphere will continue to cause warming for decades even if no more is put into the atmosphere by human activity. The climate system has inertia.
    1. Warmer and fewer cold days and nights over most land areas are virtually certain (IPCC, 2007).
    2. Warmer and more frequent hot days and nights over land are virtually certain (IPCC, 2007).

      Knowledge of the climate system together with model simulations confirm that past changes in greenhouse gas concentrations will lead to a committed(continuing) warming and future climate change... Committed climate change due to atmospheric composition in the year 2000 corresponds to a warming trend of about 0.1°C per decade over the next two decades, in the absence of large changes in volcanic or solar forcing.
      From IPCC (2007).

Thus, overall, climate forecasts for the next few decades may be accurate enough that they should be taken seriously. The Precautionary Principle should guide our actions.

Additional Resources

For more information read the pdf files of the IPCC Working Group 1 Assessment Report 4 of 2007 and the following chapters from the third assessment report of 2001, available in html format. The Executive Summary from Chapter 8 of the IPCC Third Report on Accuracy of Models of Climate Change and the Executive Summary for Chapter 9 on Projections For Future Climate Change.

Reference

IPCC (2007) Intergovernmental Panel on Climate Change. Technical Summary. Report of Working Group I: The Physical Science Basis of Climate Change. World Meteorological Organization and United Nations Environment Programme.

Vecchi, G. A. et al. (2008). Examining the tropical Pacific's response to global warming. EOS Transactions of the American Geophysical Union 89 (9): 81, 83.

Revised on: 9 February, 2009

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