Modeling the Climate System
This Much We Know
- Greenhouse gases keep earth warm.
- Greenhouse gas concentration is increasing, mostly a as a result
of human activity, including deforestation and burning of fossil fuel.
- Earth's climate is warming due to increasing greenhouse gas concentrations
in the atmosphere.
- 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
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:
- Average surface temperature of earth will warm by 2°– 6°C.
- 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.
- The answer depends on economic, political, and geological factors.
- 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.
- The Intergovernmental Panel on Climate Change has proposed
several possible scenarios. The most widely used is called Business
- 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.
- The forecasts also assume no global disasters such as a world
war or a global pandemic killing many people.
- 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?
- 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?
- Will oil become too expensive to use in cars or power plants?
- Will we substitute coal or natural gas for oil?
- Will we begin to use methane hydrates?
- Because of this uncertainty, projected concentrations in the year
2100 range from 400 to 1200 ppm (parts per million).
Are climate models sufficiently complex? The best answer is: We don't
- Thus they must include atmospheric, land, and ocean components.
- They should resolve the important variability in time and space.
- And, they must run for hundreds of years.
- But detailed, complex models cannot run for long times. They cannot
include all important processes. Compromises are needed.
- Many important climate processes are not well known.
- We don't know enough about how clouds will influence reflected
solar radiation (sunlight).
- We know little about aerosols (microscopic particles in the air).
- How do they influence absorption and reflection of sunlight?
- How many, and what type will be emitted by human activity
- We don't know if solar activity will change, and how the change
will influence climate.
- We don't yet know or understand all the important feedbacks in
the climate system. Here are just a few oceanic examples.
- If ocean surface waters warm, will primary productivity
by phytoplankton change?
- Will winds change? If they do, will they transport more or
less iron to the ocean as dust particles?
- If iron transport changes, will primary productivity change?
- 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?
- 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).
- Will the change in stability influence the deep circulation?
- Will changes in the deep circulation lead to abrupt climate
- 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
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
Group 1 Technical Summary.
We can gain some insight into the possible accuracy of climate models
by observing how well they predict past change.
- Present models cannot determine how the tropical Pacific will respond
to global warming.
- 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.
- 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.
- Climate models have predicted the observed warming over the past
It is perhaps fair to say that future predictions will not be accurate.
- Some future processes cannot be modeled. We cannot predict:
- 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."
- Volcanic activity.
- Future changes in solar activity.
- 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.
- We will never be able to test the predictions.
- We have only one earth and one future.
- 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
- Some areas, mostly polar regions will be much warmer. This is already
- We do not know how much temperature will rise in smaller areas and
in populated areas.
- We do not yet know how climate change will influence the distribution
of rainfall, tropical storms, and droughts.
- We have only imperfect understanding of how warming will change ecosystems.
- The consequences are not all bad. Overall, economists estimate the
benefits will be about the same as the costs. For example:
- Canada, New England, and Russia benefit from warmer winters
and a longer growing season.
- But, in other areas the use of air conditioning in summer will
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.
- Is a third world war likely? Not really.
- Will there be a global pandemic that greatly slows global carbon
emissions? Possibly, but not likely.
- Will economies continue to grow at the present rate? Maybe, but not
in the next few years.
- 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.
- Do we understand all feedback mechanisms in the earth system? No,
but we probably understand the most important feedbacks.
- 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).
- 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.
- Warmer and fewer cold days and nights over most land areas
are virtually certain (IPCC, 2007).
- 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.
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.
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.
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.
9 February, 2009