"Uncertainty"

How do we incorporate uncertainty in thinking about climate change?

We wish to learn:

What is the present understanding of the potential ecological impacts of global change?
What are the levels of uncertainty associated with our understanding of these impacts?

11/20/2002

Introduction

It is well known that greenhouse gases are found in our atmosphere. Naturally occurring greenhouse gases such as water vapor and carbon dioxide cause heat to be retained at the surface of the earth. Without these gases, the average surface temperature of the earth would be 0 degrees Fahrenheit. However, due to the presence of these naturally occurring greenhouse gases the average surface temperature of the earth is 60 degrees Fahrenheit.

However, human activity especially in the last 100 years is causing some greenhouse gases to be increasing in the earth's atmosphere; some examples include:

Greenhouse Gases:	CO₂	CH₄	CFC
Rate of Annual Accumulation in the Atmosphere:	0.5%	0.9%	<4.0%

For CO2, the main emissions to the atmosphere are the burning of fossil fuels (the largest source) and tropical deforestation (much smaller source) or other land-use changes.

As discussed in previous lectures, it is widely accepted that for the past ~100 years the average annual surface temperature of the earth has been rising. The total rise in 100 years has been near 1 degree F. However, the rise has not been a smooth upward trend. There was a gradual upward rise from the end of the 19th century until ~1940. Between 1940 and the early 1960's, there was a small decline in average annual earth surface temperature. The cause of this decline is not well understood. From the early 1960's to the present, there has again been a gradual upward trend in earth surface temperature.

It is also widely accepted that for the past 100 years the carbon dioxide content of the earth's atmosphere has been rising. In the early part of the 19th century the atmospheric CO₂ level was about 275 ppm (parts per million), much as it had been in previous centuries. Once we entered the era of the Industrial Revolution, as the human population of the earth began to rapidly increase in size there was also a rapid increase in the burning of fossil fuels, releasing large amounts of CO₂ into the atmosphere. Today the CO2 concentration is about 370 ppm.

Computer models of the growth of the human population and of the continued use of fossil fuels for energy suggest that it is likely that atmospheric CO2 levels will double to ~700 ppm near the end of this century. The CO₂ levels may rise well beyond that level in subsequent decades and centuries if our inputs of CO2 to the atmosphere are not curtailed.

Predicting the Future: A Major Problem

Although the past increases in greenhouse gas concentrations and the role of humans in these increases (since the 1850s) is no longer debated among the scientific community (i.e., high certainty), it is more difficult to predict what will happen to our atmosphere and climate in the future. re Computer models called GCMs (Global Climate Models) are used to determine the relationships between the energy fluxes at the earth surface and the resulting weather and climate patterns. The computer models have to account for the many variables that determine the weather over the entire surface of the globe. These variables include:

Incoming sunlight
Reflected sunlight
Outgoing longwave energy
Levels of greenhouse gases
Distribution and types of clouds
Amounts of evaporation
Distribution of water vapor
other variables related to land cover and vegetation type

The problem is that existing computers are inadequate to do more than make a rough estimate of the effect of these variables on global patterns of weather. Also the scientific understanding of how the atmosphere works needs further development. However, even with these uncertainties the current GCMs suggest that there will likely be a several degree rise in the average annual earth surface temperature if the CO₂ level doubles in the atmosphere.

Such a temperature rise would have associated changes in other properties of weather and climate. For example, evaporation rates would likely increase, length of growing seasons would change, and the frequency of extreme weather events would likely change. However, because of the limitations of the computer models, it is very difficult to speculate on the magnitude of these changes. One thing that is a virtual certainty is that the changes in weather and climate will not occur in with the same magnitudes, or even in the same direction, in different geographic regions of the globe. This is one of the challenges for scientists, to make better predictions of what kinds of ecological and societal impacts will result from future climate change that may be spatially variable.

Uncertainties in Projections of Human-Caused Climate Warming
One way to deal with these uncertainties is to categorize them into common groups. For example, the the following points were adapted from Mahlman (1997).

Highly Certain Facts

Humans are increasing greenhouse gases
Greenhouse gases heat the planet
Changed amounts of greenhouse gasse have long lasting effects on climate
Other substances (e.g. sulfates) cool the planet
CO2 increases and stratospheric ozone decreases have cooled the stratosphere about 1 degree C
In the last 100 years, the earth's surface has warmed about 0.5 degrees C

Highly Certain Projections (99% chance of being correct)

As CO2 increases, the stratosphere will continue to cool.
Water vapor will continue to increase in the Earth's lower atmosphere (6% increase for each 1 degree C of temperature rise.)

Probable projections (67% chance of being correct)

As summer temperatures increase in Northern continents at mid-latitudes, soil moisture content will decline.
At high latitude, increased precipitation (or freshwater runoff) may reduce ocean salinity, and inhibit the deep water circulation.
As sea surface temperatures rise, some tropical storms may become more intense.
As global temperatures rise, there will be an increased probability of high temperature episodes (heat waves) and a decreased chance of low temperature events (cold waves).

In addition, more recent compilations of uncertainty in climate predictions and their resulting impacts are available through the Inter-governmental Panel on Climate Change (IPCC) ( http://www.ipcc.ch/ ).

Reference

Mahlman, J.D. 1997. Uncertainties in projections of human-caused climate warming. Science 278: 1416.