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However, let me say at the outset: None of the uncertainties I will discuss can make current concerns about greenhouse warming go away. This problem is very real and will be with us for a very long time.

I will give my evaluation of current model predictions of climate change in the middle of the next century using simple "betting odds". By "Virtually Certain", I mean that there is no plausible alternative; in effect, the bet is off the books. "Very Probable" means I estimate about a 9 out of 10 chance that this will happen within the range predicted; "Probable" implies about a 2 out of 3 chance. "Uncertain" means a plausible effect, but which lacks appropriate evidence. Essentially, I set the odds; you choose your bet. My predictions follow in decreasing levels of confidence.

Human-Caused Increasing Greenhouse Gases (virtually certain)

There is no real remaining doubt that increasing greenhouse gases are due to human activities. Clear contributors are the burning of fossil fuels and the use of chlorofluorocarbons.

Radiative Effect of Increased Greenhouse Gases (virtually certain)

Greenhouse gases absorb and reradiate infrared radiation. Independent of other factors, this property acts to produce an increased heating effect on the planet.

Large Stratospheric Cooling (virtually certain)

The combination of reduced ozone and increased carbon dioxide will lead to a

marked lowering of upper stratospheric temperatures, within the range of 15o. 30° fahrenheit.

Long Time to Draw Down Excess Carbon Dioxide (virtually certain)

We know that it takes decades to centuries to produce the buildup of greenhouse gases. Much less appreciated is that a "return to normal" from high carbon dioxide levels would require many additional centuries.

Global Surface Warming over the Past Century (virtually certain)

The observed warming in the surface temperature records of about one degree fahrenheit cannot yet be unambiguously ascribed to greenhouse warming. However, no other hypothesis is nearly as credible.

Natural Variability Adds Confusion (virtually certain)

Climate varies naturally on time scales from months to centuries. This effect makes a greenhouse warming interpretation of the climate record much more

difficult.

Global-Mean Surface Warming (Very Probable)

For the middle of the next century, global-mean surface warming is estimated to be in the range of 2 to 6° fahrenheit. The largest uncertainty is due to the effects of clouds. My own opinion is the warming is more likely to be in the lower half of this range, mainly due to cooling offsets from sulfate aerosols and ozone depletion.

Global-Mean Precipitation Increase (Very Probable)

As the climate warms, the rate of evaporation should increase, leading to an increase in global-mean precipitation. Nevertheless, some local regions would experience decreases in precipitation.

Reduction of Northern Sea Ice (Very Probable)

As the climate warms, total sea ice should decrease in high latitudes of the Northern Hemisphere. However, recent atmosphere-ocean models calculate a resistance to climate change at high Southern Hemisphere latitudes. Thus, little change in sea-ice cover may occur there over the next century.

Rise in Global Mean Sea Level (Very Probable)

A further rise of 4-12 inches in mean sea level by the year 2050 is estimated

due to thermal expansion of warmer sea water. Continued sea level rise is expected for many centuries, probably to much higher values,

Summer Mid-Continental Dryness and Warming (Probable)

Model studies predict a marked decrease of soil moisture and elevated temperatures over summer mid-latitude continents. This result is sensitive to model assumptions about land-surface processes and projected levels of sulfate aerosol pollution.

Reduction of Ocean Overturning (Probable)

The predicted increase in precipitation in high northern latitudes would reduce the salinity, and hence the density in the oceans there. This effect slows down the tendency for water to sink, thus suppressing the overturning circulation.

Modeling of Cloud-Radiation Feedbacks (uncertain)

Some progress is being made in modeling the cloud-radiation response to increasing greenhouse gases. However, understanding the cloud response is still the most serious barrier to more confident predictions about climate warming.

Increased Tropical Storm Intensities (uncertain)

Some calculations suggest that a warmer, wetter atmosphere could lead to increased intensities of tropical storms, such as hurricanes. Presently, this has not been properly addressed due to inadequate computer power and to uncertainties in regional climate change predictions.

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Regional and Temporal Details of the Next 25 Years (uncertain)

I have described calculated changes averaged over decades in the middle of the next century. The predicted warming up to now is not yet large compared to natural climate fluctuations. Such natural fluctuations will continue to occur. On these shorter time scales, the natural fluctuations can artificially reduce or enhance apparent measured greenhouse warming signals.

Even though these uncertainties are daunting, important advances have already been achieved in observing, understanding, and modeling the climate. Today's models can simulate many aspects of climate and its changes. Indeed, it was these advances that led to today's increased awareness of climate change and its implications. This awareness has, in turn, escalated the need for more reliable climate predictions.

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