We will begin with estimates of the climate sensitivity, AT2x. The IPCC Third Assessment Report (2001) (IPCC as an organization is described in Chapter 13) reports that the mean of the climate sensitivity, AT2x, of 15 climate models is 3.5°C. The standard deviation of the climate sensitivities, denoted by the Greek letter sigma (a), is 0.9°C.

A standard deviation describes the variability or spread of a list of numbers, in this case model AT2x values. If the values are distributed in a normal "bell" curve, then the meaning of the standard deviation would be that 63% of the AT2x estimates should fall within 1a of the mean (in this case ±0.9°C), and about 95% of the estimates would fall within 2a (± 1.8°C). The statistics imply that with 95% certainty, a new estimate of

AT2x, such as from some new model, should probably fall between 1.9°C and 4.1°C. This range of model results in one measure of how well we can forecast the future.

Another approach to estimating how well we know A T2x uses a bit more brute force. Climate models have knobs, numbers that are not known precisely which affect the behavior of the model. These numbers are called tunable parameters. Cloud droplets, for example, do not form immediately when the relative humidity exceeds 100%. They may start forming at 110% relative humidity, or 120%. The best value of this parameter to use in a model is not known precisely. It varies from cloud to cloud, no doubt. Our model does not resolve all of the processes that would enable it to predict cloud droplet formation. The model must be told what value to use.

A brute force method for estimating the uncertainty in the model forecast is to run a climate model many times using a range of values of many different tunable parameters, varying multiple parameters at the same time. Climateprediction.net uses donated "screen saver" computer time to tackle this job. Each set of model parameters was run multiple times, with slightly varying initial conditions, to generate an ensemble (Chapter 7). Stainforth et al. (2005) analyze over 2000 model runs, over 100,000 years of model time. The URL for this project is www.climateprediction.net. Perhaps you would like to contribute to the effort yourself.

The distribution of model runs is shown in Fig. 12.1a. Most of the model combinations predicted a AT2x of about 3.4°C. There is a long "tail" to the distribution (this is not a normal "bell" curve), with a few very high AT2x estimates, ranging to 11°C. On the other side, there were very few parameter settings that came up with A T2x of much less than 2°C.

Each run included a control period, where CO2 concentration remains constant, and a period with doubled CO2. The climate of the control period was compared with meteorological data, and all the misfits added up into a single number. The misfit numbers from the runs are compared with the errors of a hand-tuned ("standard") model run. The misfits from the low-AT2x models tended to be a bit higher than the error from high-AT2x models (Fig. 12.1b) but none of the errors was bad enough to declare any of the models to be obviously wrong.

Uncertainty in A T2x is only the beginning of the uncertainty in the temperature forecast for the coming century. AT2x gauges the equilibrium response to doubling CO2, but it takes quite some time for the climate system to reach equilibrium. Predicting the temperature in the year 2100 requires modeling what is called the transient response. IPCC uses a standard benchmark for comparing the transient responses of models, which they call Transient Climate Response (TCR) and define as the model temperature at doubled CO2 concentration when the CO2 has been rising at a rate of 1% per year (Fig. 12.2). Real atmospheric pCO2 itself is not rising at 1% per year, but the idea is to raise CO2 a bit faster, to account for the greenhouse forcing from methane and other greenhouse gases. If the radiative forcing in the future doesn't go up as fast as 1% per year, then the TCR would be more appropriate to compare with the radiation = doubled CO2 year, whatever year that turns out to be. The average TCR values from 20 models is 1.8°C with a standard deviation of 0.4°C.

Climate takes a long time to change, that is to say, it has a long transient response to the change, in part because the ocean stores a lot of heat. During this time when we are



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Worse fit

Same fit

Fig. 12.1 Results from ClimatePrediction.net. (a) Distribution of the climate sensitivity, A T2x, of different model configurations. (b) Error in predicting the real climate, relative to the error of a hand-tuned model = 1.0.

waiting for the Earth to warm up, there is an imbalance in the energy budget of the Earth. First a rise in greenhouse gas concentration decreases the outgoing flux of heat, and then eventually the Earth warms up, ramping up its outgoing energy flux, until the Earth's energy budget finds its new balance.

The energy budget of the Earth today, based on satellite measurements, is out of balance by about 0.75 W/m2. The excess heat is being absorbed into the ocean. We can measure that the temperature is rising in the upper ocean, and calculate that the ocean is where most of the 0.75 W/m2 of heat is going. As the ocean warms, it allows the climate of the Earth to warm, increasing the energy flux back to space, until the energy budget of the Earth reapproaches a state of balance (Fig. 12.3). It will take centuries to warm up the oceans to a new equilibrium temperature distribution. The oceans are keeping us cool.

The warming from rising greenhouse forcing takes a long time also because of feedbacks in the climate system, such as the ice albedo and the water vapor feedbacks, discussed in Chapter 7. The effect of these feedbacks is to make it harder for the poor old Earth to balance her energy budget. I won't guarantee it, but it might be helpful to look at Fig. 12.3 again. An increase in temperature of the Earth increases the water vapor concentration, a greenhouse gas that blocks infrared energy loss to space.

Fig. 12.2 TCR is a diagnostic for comparing one model with another. It is a snapshot of the temperature at the time when CO2 reaches double the preanthropogenic value. If CO2 is held constant at that level, the climate continues to rise, until, in equilibrium, the temperature reaches the climate sensitivity, A T 2x.

Fig. 12.2 TCR is a diagnostic for comparing one model with another. It is a snapshot of the temperature at the time when CO2 reaches double the preanthropogenic value. If CO2 is held constant at that level, the climate continues to rise, until, in equilibrium, the temperature reaches the climate sensitivity, A T 2x.

Earth's energy balance

Solar in Infrared out

— With feedback


Earth's temperature

No feedback With feedback


Fig. 12.3 The effect of an amplifying feedback is to prolong the transition to a new climate, as well as increase the final temperature change. (a) Earth's energy balance and (b) Earth's temperature.

The outgoing energy flux does not increase as much with rising temperature (gray solid line in Fig. 12.3) as it would if there were no feedbacks (black solid lines in Fig. 12.3). The existence of the feedback slows down the approach to the new climate equilibrium, as well as making the final temperature change larger.

So the amount of time it will take to balance the energy budget depends on two things. One is the heat uptake by the ocean, and the other is the strength of the feedbacks such as water vapor. One estimate of the equilibration time for climate is about 60 years. If the real feedbacks turn out to be stronger than we think, this will have two consequences. One is that the ultimate temperature increase will be greater, and the other is that it will take longer to reach the ultimate temperature increase. The best guess is that about 40% of the warming that will occur from the CO2 already released, what is called committed warming, has yet to take place. We have paid for 1 °C warming, but we have so far received only 0.6°C.

Transient climate runs forced by the IPCC BAU scenario predict temperatures 2-5 °C warmer by the year 2100 (Fig. 12.4). The uncertainty in our forecast for the temperature in the year 2100 derives from two sources, which contribute about equal to the uncertainty. One is the uncertainty in model temperature response to a given amount of CO2, and the other is uncertainty in what our CO2 emissions will be in the future.

A temperature change of 2-5°C may not sound like very much. The daily cycle of temperature is greater than that, to say nothing of the seasonal cycle of temperature. The main impacts of future climate change may come from changes in rainfall, rather than temperature. But the temperature change by itself is far more significant to the landscape of the world than you might think.



1850 1900 1950 2000 2050 2100

1850 1900 1950 2000 2050 2100


Fig. 12.4 Model simulations of the BAU scenario, from IPCC 2001.

One point of comparison is the temperature difference between now and the last ice age, estimated to be about 6°C. This was a huge climate change. If we were looking forward to a Glacial Maximum in the future, rather than backward into the safe past, we would be in a panic. It would be apocalyptic. Europe was an occasionally habitable tundra. All the vineyards and beer gardens, forget about them. The ice sheet in North America dwarfed what exists today in Greenland. Greenland, in spite of its name, is a pretty fierce place. Of course, Europe and North America were extreme cases because the ice sheets were here, but the landscape looked different around the globe. Changing snowlines make it clear that it was noticeably colder. Pollen data show huge changes in vegetation type. The coastlines were barely recognizable. Truly it was a different world.

Another comparison is to the Little Ice Age and Medieval Warm periods (Chapter 11). Some reconstructions of the global mean temperature, or Northern hemisphere temperature, show temperature changes of perhaps 0.5-1°C. These climate intervals were not the end of the world, but they definitely sufficed to rearrange civilizations. In medieval time, European agriculture was a bounty of plenty, in a stable, benign climate. Meanwhile a 500-year drought coincided with the demise of two organized civilizations in the New World, the Classic Maya and the Anasazi. The Little Ice Age climate was much more unstable than it was in medieval times. Temperature or rainfall would change suddenly for a year, or decades. There were periods of drought, periods of hot summers, of arctic winters, and of mild periods of moderate climate and good harvests. The years 1690-1730, roughly coincident with the Maunder Minimum, saw sea ice around Britain and northern France and a complete rearrangement of the fisheries in the Atlantic.

The impression I have is that a temperature change of 1°C is probably not a world-shattering change, at least globally, although there is the risk of essentially perpetual regional droughts, such as occurred in the American Southwest during the medieval warm time. By analogy to the intensity of the climate changes that came with the end of glacial time, I would guess that a global mean temperature change of 5°C would be catastrophic.

The distribution of the forecast temperature change is not uniform geographically or in time. Plate 12.1 shows the mean annual temperatures from a climate model described in the Projects section and in Bala et al. (2005). The atmosphere model was developed at the National Center for Atmospheric Research, a government agency in Boulder, Colorado. The ocean model was developed at the Los Alamos National Lab in New Mexico. In general, this particular model has a relatively low climate sensitivity, AT2x, of 2-3°C for doubling CO2. The temperatures are plotted as anomalies, differences from the temperatures in year 2000 in Plate 12.2.

The high latitudes warm more than low latitudes, by a factor of 3 or 4, because of the ice albedo feedback. Winter temperatures in Alaska and Western Canada have warmed by 3-4°C, compared with a global average of perhaps 0.5°C. Much of the high-latitude land surface is permafrost, defined as soil that is frozen year round. The surface of the soil may melt in summer, a layer in the soil called the active zone. As temperatures rise, the active zone gets thicker. As subsurface ice melts, the soil column collapses, leaving houses and trees tilted at crazy angles. Most of the Trans-Alaska oil pipeline has its foundation in permafrost soil. Lakes suddenly drain away, as melting ice leaves holes in the soil column. Coastlines are collapsing at rates of 40 m/year in some parts of the Canadian Arctic and Siberia. Models predict that the tundra ecosystem may disappear almost entirely in the coming centuries.

In mid-latitudes and the tropics, the day-to-day impact of the temperature change may be more subtle in most places. Wintertime and nighttime temperatures will warm more than summer daytime temperatures. This is because higher CO2 acts to decrease radiative heat loss from Earth's surface to space. Greenhouse gases not only warm the planet in general, but they also hold the heat in longer during the cold times. All seasons tend to warm, but cool times get a double whammy.

Land tends to warm more than water because evaporation may carry away heat from the water, but the land may dry out. Some models predict a general drying out of continental interiors for this reason. Growing seasons will get significantly longer. Growing seasons are already about a week longer than they were a few decades ago.

There will be more days of extreme heat, and fewer days of extreme cold. Projections of mortality from heat waves show an increase in heat-related deaths. It must be said however that there are also projections of mortality from cold, which decrease. No doubt the residents of tropical cities like New Delhi, already roasting in tropical urban heat islands, will not welcome a further few degrees of heat. A two-week heat wave in Europe in August 2003 is estimated to have killed 35,000 people. Canadian farmers on the other hand may find advantage in the new climate, with longer growing season and milder winters. The projections of economic impacts of climate change show winners as well as losers for small changes in climate, while for large climate changes almost everyone loses.

The warming due to CO2 is offset by cooling from sulfate aerosols, but the amount of cooling from the aerosols is not the same everywhere. The CO2 radiative effect is pretty much the same everywhere, proportional to local temperature, because CO2 is a well-mixed gas in the atmosphere. Aerosols, on the other hand, last only a few weeks before they are removed from the atmosphere as acid rain. The cooling effect of the aerosols is therefore concentrated near the sources of their release, predominantly in the industrialized northern hemisphere.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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