Climate Models & Predictions for the Future

"Time drops in decay,
Like a candle burnt out,
And the mountains and woods
Have their day, have their day".
-W. B. Yeats, "The Moods"

We will learn:

How has the climate changed during the recent past
How do we describe the current climate?
What can we say about potential future climate change?
How do climate models work and what are their predictions for the future?

Introduction To Global Change I Lecture Notes			Format for Printing
Short-Term Paleoclimate	Current Climate	Climate Models	Summary

1. Short-Term Changes in Paleoclimate

In the last two lectures we discussed paleoclimate over very long time scales. The broad, glacial-interglacial variations observed over the 150,000 year record shown in Figure 1d are explained mainly by solar forcing due to the Milankovitch theory.

Figure 1. The Paleoclimate Temperature Record
at various time scales. Changes are deduced using a number of paleoclimate techniques.

Shorter term changes in climate, such as the Younger Dryas period shown in Figure 1c, was found to be related to how the formation of North Atlantic deep water and the resulting Conveyor Belt circulation has changed over time. However, it remains to be explained how the observed correlation between the greenhouse gases methane and carbon dioxide as measured in the ice cores, and the temperature shifts during the last several glacial periods, are actually related.

The correlation between methane, CO₂ and temperature could perhaps be explained by invoking a temperature dependence to the cycling of CO₂ and methane through the environment. For example, the main reservoir for CO₂ is the ocean. A dependence of the flux from the ocean on temperature, such that the higher the temperature the greater the flux to the atmosphere, could explain the correlation for CO₂. In addition, we know that as temperature warms the oceans can hold less dissolved gas, which would be consistent with the idea that the gas would leave the ocean and reside in the atmosphere. A similar temperature dependence for the decomposition of organic matter, which can produce methane in anaerobic conditions (see later lecture on Microbes), could explain the methane correlation.

As mentioned previously, one thing that we do know for sure is that the physics of heat-trapping gases are unavoidable -- that is, if we increase the concentration of these gases in the atmosphere, they will trap more heat and we will warm the atmosphere. These relationships between heat-trapping ability and gas concentration forms the basis for many of the predictions of future global climate, as discussed below.

2. Current Climate

Before moving on to the future climate, we need a better understanding of Earth's current climate. Climate differs from weather in that it provides a statistical view of seasonal and daily weather events over a long term period. Thus, for example, the passage of a large rain storm in a frontal system over Ann Arbor is weather event, while the average number of such passages for the month of July (averaged over several years) is part of the climate record. So, climate is the long-term average of weather.

Climate records are most often expressed in terms of temperatures, winds, precipitation, and pressures - all parameters that can be measured at multiple sites around the globe. Over the years a large data base of weather event measurements has been obtained, leading to a good description of today's climate.

We find that climate varies widely around the globe - we have deserts and rain forests, ice caps and "death valleys". As for most subjects discussed in this course, there is a taxonomy of sub-disciplines and we can speak of the following:

Figure 2. Precipitation and temperature seasonal averages.

Microclimate: climate conditions near the surface over distances of a few meters. Great perturbations to the microclimate can rapidly affect plant life.
Mesoclimate: climate conditions over a few square kilometers, for example, climate of a town, valley, large beach, etc.
Macroclimate: climate conditions for a state or a country - over scales ~1000 km or greater.
Global Climate: The overall climate of the planet. We have already discussed, for example, the mean surface temperature and its variations with geologic time.

The many factors that control local climates include: intensity of overhead sun - including its latitudinal variation; the distribution of land and water; ocean currents; prevailing winds; positions of semi-permanent high- and low-pressure areas; mountain barriers; and altitude. The effects of these controls can be seen in global patterns of temperature and precipitation. Examples of local climatic data are shown in Figure 2. These graphs are derived from temperature and precipitation data available from the National Climate Data Center. The climate data for Detroit is listed in Table 1.

Great differences in climate occur from place to place, even within the continental United States which only accounts for about 2% of the Earth's surface. In 1918, a popular climate terminology was developed by Koppen and is called the Koppen System. It is based on annual and monthly average temperature and precipitation measurements, using evidence from vegetation where data is sparse.

Figures 3 uses this system to provide a summary of the types of climates found on today's Earth.

Figure 3. The Koppen System of determining the major climate zones on Earth. The key elements of the classification are temperature and moisture.

TABLE 1. Climatological monthly temperatures and precipitation for Detroit, Michigan.

	JAN	FEB	MAR	APR	MAY	JUN	JUL	AUG	SEP	OCT	NOV	DEC	ANNUAL
Temperature (°F)	22.9	25.4	35.7	47.3	58.4	67.6	72.3	70.5	63.2	51.2	40.2	28.3	48.6 (MEAN)
Precipitation (in)	1.76	1.74	2.55	2.95	2.92	3.61	3.18	3.43	2.89	2.10	2.67	2.82	32.62 (TOTAL)
Data from the National Climate Data Center

Global Temperatures

Figure 4. Average annual sea level temperatures throughout the world (degrees F)

Up to now, we have discussed the global average temperature. Actual local temperatures can differ greatly from the global mean, being generally warmer at lower latitudes and colder at higher latitudes. Figure 4 illustrates the annual sea level temperature (in degrees F). The lines are contours of temperature (called isotherms) and are generally oriented east-west. The primary changes are in latitude, with the equatorial region being the hottest due to the extra sunlight absorbed there.

Note how the isotherms tend to bend along coastlines. This is due to the unequal heating of land and water and the tendency of the winds to blow along coastlines, creating the upwelling and downwelling discussed in the last lecture. Also of significance are the ocean currents both on the surface and where the deep-water currents rise to the surface. Examples of ocean currents include the California Current which flows southwards along the Californian coast and the Gulf Stream which flows northwards in the Atlantic.

Global Precipitation

Figure 5 summarizes the modern global mean precipitation climate record. Notice the high degree of regional variability.

Figure 5. Global patterns of precipitation are closely tied to general atmospheric circulation and topographic changes, as modified by the upwelling of cold ocean currents such as on the west coasts of South America and southern Africa.

3. Current Trends in Climate

The global average surface temperature has increased by 0.6 ±0.2°C since the late 19th century (IPCC, 2001). It is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record since 1861 (see Figure 6).

As indicated in Figure 6, most of the increase in global temperature since the late 19th century has occurred in two distinct periods: 1910 to 1945 and since 1976. The rate of increase of temperature for both periods is about 0.15°C/decade. Recent warming has been greater over land compared to oceans; the increase in sea surface temperature over the period 1950 to 1993 is about half that of the mean land-surface air temperature. The high global temperature associated with the 1997 to 1998 El Niño event stands out as an extreme event, even taking into account the recent rate of warming.

Figure 6: Combined annual land-surface air and sea surface temperature anomalies (°C) 1861 to 2000, relative to 1961 to 1990. Two standard error uncertainties are shown as bars on the annual number [from IPCC, 2001]

It also appears that the spatial patterns of warming that occurred in the early part of the 20th century were different than those that occurred in the latter part. Figure 7 shows the regional patterns of the warming that have occurred over the full 20th century, as well as for three component time periods. The most recent period of warming (1976 to 1999) has been almost global, but the largest increases in temperature have occurred over the mid- and high latitudes of the continents in the Northern Hemisphere. Year-round cooling is evident in the northwestern North Atlantic and the central North Pacific Oceans, but the North Atlantic cooling trend has recently reversed.

| 1901-2000 |
| 1910-1945 | 1946-1975 | 1976-2000 |

Figure 7. Temperature trends for the periods 1901-1999, 1910-1945, 1946-1975 and 1976-1999. Trends are represented by the area of the circle with red representing increases, blue representing decreases, and green little or no change. [From IPCC, 2001]

Modern Changes

Modern changes in temperature and carbon dioxide are shown in Figures 8 and 9. It is clear from these figures that rapid changes are underway - at rates far exceeding anything discussed so far.

It must be assumed that human activities (known as Anthropogenic Effects) are dominating the present changes. In fact, a detailed analysis of the Earth's carbon cycle (discussed in an upcoming lecture) indicate unequivocally that these changes in the atmosphere are due to human activities and not to some random variation in time.

The past century has seen an increase in the global mean temperature of 0.8^oC., and nearly all of the top 10 warmest years on record have occurred in just the past decade.

The upward trend may also be a consequence of the increasing levels of carbon dioxide, methane and CFCs put into the atmosphere through various anthropogenic processes.

Figure 10 shows multi-year measurements of carbon dioxide abundances in the atmosphere from a single location in Hawaii. This is the longest continuous record of accurate CO₂ measurements that exists. The strong yearly signal (peak in winter months, minimum in summer months) can be seen, together with the strong overall increase due to anthropogenic effects. The causes and consequences of this rise will be the subject of much additional discussion in this course.

In Figure 11 the modern records for atmospheric methane and carbon dioxide are shown in a slightly different form to highlight the seasonal differences and the latitudinal differences. Both gas species show a strong yearly cycle (peaking in the winter months). The cycle is more pronounced in the Northern Hemisphere (toward the top of the graphs) than in the Southern Hemisphere, where the seasonal variations are muted.

The reason for the high carbon dioxide and methane concentrations during winter months is that during the winter land plants are much less active, and the balance of photosynthesis (uptake of CO₂) to respiration (production of CO₂) leans toward respiration. This CO₂ accumulates in the atmosphere and causes the peak in the winter months.

In the summer months the opposite occurs, and the plants are active and they remove CO₂ from the atmosphere for photosynthesis. This results in a lower concentration in the atmosphere, and the corresponding valleys in the graph (Figure 11).

The second aspect of these graphs is to notice the big difference in atmospheric concentrations and seasonal variations between the northern and southern hemispheres. Most of the land mass of Earth is in the Northern Hemisphere, and thus most of the CO₂ and methane are produced there. In addition, there is a preponderance of human activities in the Northern Hemisphere, which also contributes to the production of these gases.

Overall, most of the recent increases in global temperature are due to greenhouse gas increases in the atmosphere. Some of the variations, however, are a consequence of volcanic eruptions. Such events can emit large quantities of dust into the stratosphere where sunlight can be intercepted for a period of a few years.

Finally, the idea that changes in the number of sunspots can affect climate is hotly debated. Although there were fewer sunspots during the Little Ice Age than there are now, it remains to be shown that the power (heat) generated by the sunspots is really large enough to control temperatures on Earth.

Figure 8. Modern variations in CO₂ and temperatures.

Figure 9. Recent changes in global mean air temperature. Top panel Northern Hemisphere. Bottom panel Southern Hemisphere.

Figure 10. Multi-year climate record of atmospheric CO₂ concentration in Hawaii.

Figure 11. Multi-year and latitudinal variation of CO₂ (top) and CH₄ (bottom).

4. Climate Models

The Intergovernmental Panel on Climate Change has released a series of Technical Summaries in 2001. Download:

1. "Climate Change 2001: The Scientific Basis" (especially germane to this topic)

2. "Climate Change 2001: Impacts, Adaptation and Vulnerability"

3. "Climate Change 2001: Mitigation"

A number of sophisticated global climate models have been developed over the past 20 years for the purpose of predicting future climatic change. The most highly developed models are three-dimensional and time dependent and divide the globe up into a series of interacting boxes. The reservoirs and fluxes of importance are coded into the computer program which then solves the conservation equations of mass, momentum, and energy in order to calculate the evolving state of the atmosphere/hydrosphere system.

In general, models such as these must be validated against observations. This is done sometimes by running the model backwards in time to specify past, known climates. Often the models are run starting with input data for the year 1950, in part because the climate data from that time forward are particularly complete and detailed. As the model runs it outputs its "predicted" climate, which can be then easily checked against what actually happened. If the predictions match the actual observations then the model is considered to correctly simulate all of the main climatic processes and equations, and the model then continues to run into the future. If the predictions for past years don't match what actually happened, then the model is developed further. However useful, these predictive models have to constantly checked against experimental data to insure accurate results (Figure 12).

Climate models are often used to predict the climate of an Earth in which the carbon dioxide concentration has doubled. This is a prospect very likely to occur within the next 50-100 years, given the current increasing rates of anthropogenic CO₂.

Below are some results of climate models run under twice the current global carbon dioxide concentration. The model predictions for future climate are based on forward estimates of the rate of fossil fuel consumption, and thus the production of CO₂ and its input to the atmosphere.

These estimates are called SRES estimates, which stands for Special Report on Emissions Scenarios which was produced by the IPCC. These scenarios include situations such as "business as usual", where we do nothing to alter our consumption of energy on the planet. This of course results in the most drastic predictions of warming or sea level rise. Other scenarios (B, C, D, etc.) all account for various strengths of efforts that we might make to curb our use of fossil fuels.

Given these future scenarios of carbon dioxide concentration in the atmosphere, the models are run with their normal prescriptions for the multiple interactions among clouds, land, oceans, and so on in the Earth's climate system. Some results follow:

Climate models predict a climate that is significantly hotter and more humid than now.
Climate models predict an increase in the mean sea level of up to 6 meters over the next 100 years. Different scenarios give different results, but the basic trend is the same (Figure 13 - note this is realized sea-level rise).
Climate models also predict increases of 4 degrees over the same 100 year interval. It must be noted that the exact prediction is dependent on the assumptions used for fossil fuel consumption rate, or the scenarios (see Figure 14).

Figure 12. Interaction of Climate Models and experimental data.

Figure 13. Climate models predictions for sea level changes
according to several scenarios of fossil fuel consumption.

Figure 14. Climate model predictions for temperature changes
according to several scenarios of fossil fuel consumption.

4. Summary

1. The most recent changes in Earth's paleoclimate record are likely due to shifts in ocean circulation, and the effects of greenhouse gas increases. Volcanoes have had only a small effect, and the sun spot record has yet to show that this variation in heat input has been important.

2. Climate is the long-term, statistical average of short-term "weather" events, such as individual storms or droughts. The current climate sets a benchmark against which to judge changes in past, and in the future.

3. In recent times, temperature changes and greenhouse gas abundances are correlated. Rapid global warming is underway and models have been developed to predict the effects of these changes. The models are sophisticated, mathematical representations of the interaction between the land-ocean-atmosphere system on Earth, and rely on "scenarios" of future fossil fuel use in order to make predictions for the future.

4. These Global Climate Models (GCMs) predict a much altered climate on Earth during the next century. There will be substantial warming and sea-level rise, and altered patterns of precipitation with some areas receiving more and others less.

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