(37) Global Warming

Our climate models are WRONG: Global warming has slowed - and recent changes are down to ‘natural variability’, says study

What is a General Circulation Models or GCMs?

GW37

Results of Climate Models-Levels of Certainty

Predictions:

• Global average temperature. The global average temperature is expected to rise by 0.64-0.69°C (1.15–1.24°F) by the years 2011–2030 compared with the years 1980-1999. This result is the consensus of three different models used by IPCC investigators.

• Areas most affected. The greatest temperature increases will occur at high northern latitudes (such as Alaska and the Arctic regions) and over land areas.

The Arctic Ocean is likely to be free of ice during the summer months within roughly five decades. The Arctic sea ice overall is expected to lose 54 percent of its volume by the 2050s.

• Ocean circulation. Some investigators have expressed concern that the ocean circulation patterns that keep northern European countries in a more temperate range may be disrupted by the effects of global warming.

The IPCC predicts that the massive Atlantic circulation flow called the thermohaline circulation (and also referred to as the meridional overturning circulation, or MOC) “is very likely to slow down by 2100.

However, according to the IPCC, it is very unlikely that the MOC will undergo a large abrupt transition during the course of the twenty-first century”

• Ice sheet melting. At least partial melting of the Greenland ice sheet and the West Antarctic ice sheet is expected if there is a global temperature increase in the range of 1-4°C (1.8-7.2°F) compared with 1990–2000. Because of the many uncertainties associated with this outcome, the IPCC places what it calls “medium confidence” in this outcome, in contrast with other predictions that are known with a greater degree of certainty. This melting may take centuries or even thousands of years.

• Weather extremes. It is considered very likely that heat waves and heavy precipitation events will continue and become more frequent. However, observations of small-scale severe weather such as tornadoes are local and too scattered to draw conclusions from. To some extent, greater public awareness and better tracking of these events may inflate records.

• Drought. Climate data show a large drying trend over many areas of North America, southern Europe and Asia, northern Africa, Canada, and Alaska. An index used to measure the occurrence of drought conditions around the world that combines the local effects of both water shortage and elevated temperature.

• Antarctica ice sheet. Models suggest that the Antarctic ice sheet as a whole will remain too cold to melt and will gain in mass as a result of increased snowfall.

HOW ACCURATE ARE THE MODELS?

Much has been accomplished in the science of climatology in recent years. Some aspects of the earth’s climate are known with a high degree of confidence. As might be reasonably expected, there are areas where the jury is still out and further work remains to be done. Much of this centers around gaining a more precise understanding of past climate changes. In many ways, the scientists and modelers seem to be attempting the technical equivalent of herding cats. But we are far from clueless.

A standard scientific practice is to determine the accuracy of a particular set of measurements and consequently the conclusions based on those measurements. Uncertainty can be either in (1) determining a particular measured value such as temperature, sea level, or mass of an ice sheet or (2) the cause-and-effect relationships between the variables, such as how much new carbon dioxide will the oceans absorb, how much warming will result from a given amount of greenhouse gas, or how much warmer will the North Atlantic need to be to completely cause the Greenland ice sheet to melt.

Since some outcomes have a much greater degree of certainty than others, the IPCC has established a consistent terminology to use in describing climate predictions. For instance, the following statements are examples of the results of models used to simulate the effects on climate:

• It is very likely that the average northern hemisphere temperatures during the second half of the twentieth century were warmer than any other 50-year period in the last 500 years.

• It is very likely that human (anthropogenic) greenhouse gas increases caused most of the observed increases in global average temperatures since the mid-twentieth century.

• It is virtually certain that human-generated aerosols have a cooling effect on northern hemisphere air temperatures.

• It is extremely likely that human activities have exerted a warming effect.

• It is extremely unlikely that the earth would naturally enter another ice age for at least 30,000 years.

• It is likely that there have been increases in the number of heavy precipitation events.

• There is high confidence that the rate of sea level rise has accelerated between the mid-nineteenth and twentieth century’s.

POSSIBLE AND PROBABLE OUTCOMES

One of the most likely outcomes of climate change with the greatest degree of certainty attached to it is an increased incidence of warmer days and nights and a reduced incidence of cold days and nights.

BASIC UNCERTAINTIES-WHERE THE JURY IS STILL OUT

In science, it is usually valuable to know what you do not know. Defining the limitations of knowledge clarifies areas for follow-up effort and separates logical conclusions from speculation. As much as climatology has evolved in recent years, there remain the following areas of uncertainty:

1. The effect of solar intensity variability for the past few centuries is not clear because instruments to precisely make this measurement were not available at the time. Insight into the effects of changing solar intensity on past climates can only be derived from indirect or proxy data.

2. The detailed interaction between aerosols and cloud properties is not well understood.

3. Causes of the recent reduction of methane in the atmosphere are subject to various theories but are not clearly understood.

4. Details of stratospheric water vapor (such as contributed by airplane contrails) are not well quantified.

5. Some radiosonde balloon data may be unreliable, especially in tropical areas.

6. Information on hurricane frequency and intensity is limited to recently acquired satellite data. This makes it difficult for observers to determine with confidence whether there is a trend toward more severe weather.

7. Similarly, there is insufficient evidence to determine whether tornados and other severe weather are intensifying.

8. Prior to 1960, there was no global measurement of overall snow cover.

9. There is not enough information to draw a conclusion about trends in the thickness of Antarctica sea ice.

10. The global average sea level rise measured between 1961 and 2003 is larger than can be fully accounted for by thermal expansion and land-based ice melting.

11. Mechanisms for past abrupt climate change (such as the Younger Dryas Period) are not well understood. Thresholds for when abrupt changes may occur are also not nailed down with precision.

12. Historical (paleoclimate) records are available for the northern hemisphere, but fewer records exist for the southern hemisphere.

13. Factors affecting temperature changes are much better understood than those influencing precipitation.

14. Processes taking place in the ocean depths that influence climate are more difficult to model.

15. Models suggest that the thermohaline circulation (or meridional overturning circulation) will not be disrupted by the end of the century.

Greater uncertainty exists in predicting how greenhouse gas emissions will affect the MOC after 2100 and whether there is a threshold that could trigger abrupt changes.

16. The ENSO is only partially understood and is not modeled the same way by all scientists.

17. There is uncertainty about the amount of sustained global warming that would lead to complete melting of the Greenland ice sheet.

18. There is very limited correlation between climate variables and the incidence of extreme events.

19. During past ice ages, the carbon dioxide level in the atmosphere dropped.

Although various possible explanations have been proposed, the precise mechanism causing this has not been determined.

20. Perhaps the greatest uncertainty does not know what actions the inhabitants of the earth will take in response to the climatic changes that are known with higher levels of certainty.

Key Ideas

• Climate sensitivity describes how much global average temperature increases when the carbon dioxide level in the atmosphere doubles.

• If the carbon dioxide level in the atmosphere doubles, the global average temperature is expected to increase by 3°C (5.4°F) within a range of 2-4.5°C (3.6-8.1°F).

• Climate models predict climate changes based on quantitative relationships between the variables and feedback.

• Feedback occurs when the results of one process affect the outcome of a second process, which, in turn, affects the outcome of the first process.

• Positive feedback causes an outcome to be more intense than it otherwise would have been.

• Negative feedback causes an outcome to be less intense than it otherwise would have been.

• Current global warming is expected to cause disruptions to water supplies (initial excesses followed by shortages), intensified droughts and floods, and more severe weather.

• The global average temperature is expected to rise by 0.64-0.69°C (1.15-1.24°F ) by the years 2011–2030 compared with the years 1980-1999.

• Ocean circulation processes (thermohaline circulation) are expected to slow but not be disrupted entirely.

• The Arctic Ocean may have no ice at the end of the summer of 2060.

• The Antarctic ice sheet as a whole will remain too cold to melt entirely and will gain in mass as a result of increased snowfall.

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