No. We experience weather locally, at one place and at one time. As everyone knows, weather can vary enormously over the course of one year (100 degrees F or more) and between one region and another. Climatology studies the average weather of the entire globe over centuries.

Climate's natural variation has been large. Paleoclimatologists, who attempt to reconstruct the climate's of past eras, have evidence that at times the Earth's average temperature has been 10 degrees cooler than today and at other times 20 degrees warmer than today. Many natural factors (operating on very different time scales) are known to change climate: subtle variations in the Earth's orbit round the sun; the positions of the continents as the tectonic plates move; the output of the sun's light, the eruption of volcanoes, ocean patterns etc.

It's been very difficult. Climatologists only have comprehensive global temperature and rain fall measurements going back a century. Using so-called "proxys" such as corals, tree rings, ice cores and lake sediments, scientists have been able to reconstruct longer records--back about 1,000 years. Over this period, the last 100 years appears to stand out. (see graph) Since 1900, the Earth's average global temperature has warmed by about 1 degree F--hardly noticeable to most of us, but a rapid increase compared to most natural changes. Interestingly the shape of this curve is not a straight line. Much of the warming occurred early in the century, then it leveled out for three decades around mid century. This, it's argued, shows that human greenhouse gas build-up is only one factor influencing the climate. Other factors--for example, the changing output of the sun, other pollutants like sulphate aerosols which have a cooling effect and, volcanoes--also contribute to the aggregate effect.Why are many scientists and policy experts worried about global warming, even though there is no "smoking gun" demonstrating that serious human-caused global warming is underway? The main reason people are concerned is the existence of a phenomenon called the Greenhouse Effect. Trace gases in the atmosphere like carbon dioxide, methane and water vapor can trap infrared radiation escaping into space. Without such naturally occurring gases in our atmosphere, the Earth's average temperature would be 0 degrees F instead of the comfortable 59 degrees F it is today. However, too much CO2, can produce an inferno. (Venus is a good example of a planet with an atmosphere made almost entirely of CO2.) Since carbon dioxide is given off when fossil fuels like oil, coal and gas are burned, it follows that more heat energy should be trapped in the climate system causing the global temperature to increase.

Charles David Keeling's work on the mountain of Mauna Loa in Hawaii established (see graph) that since 1957 the CO2 level has risen exponentially and that the industrial CO2 diffuses throughout the entire atmosphere in about 18 months. Samples from the Vostock Ice in the Antarctic have extended the Keeling curve backwards in time, showing that CO2 has risen one third since the industrial revolution, and it is higher than at any time for the past 450,000 years.

Most of it. Each year about 7 billion tons of carbon (which turns into over 20 billion tons of CO2) are released by cars, factories and energy production (see graph). About one half of this is absorbed by the oceans and the biosphere. The rest stays in the atmosphere, joining other post-industrial revolution CO2 and remaining there for an average residence of 100 years. If things continue as they are--with the developing world rapidly industrializing--then by 2100, the world's annual emission of carbon will exceed 20 billion tons (70 billion tons of CO2) and the amount (concentration) built up in the atmosphere will be at least triple the pre-industrial level.

Not really. Greenhouse gases like CO2 and methane "force" the climate by trapping energy in the climate system. But it's just part of a very complex system involving other forcings. For example, as mentioned above, there are forcings like the sun's illumination which varies over cycles of a decade or more. Or, volcanoes which spread ash into the air and create a cooling effect. Other human pollutants like sulfate aerosols (also produced by coal burning) actually cool the climate system by reflecting incoming sunlight. But the biggest uncertainty is due to feedbacks which either amplify (positive) or dampen (negative) the effects of a forcing. The principal positive feedback is water vapor. Increased CO2 causes warming...leading to more water vapor which is itself a greenhouse gas...leading to more warming. Warmer ocean water bubbles out the dissolved CO2 which then leads to more warming, more water vapor and so on. But there are negative feedbacks also. Water vapor can form clouds which reflect incoming sunlight and thus have a cooling effect. Finally, there are delays or inertias in the system, notably the oceans which can absorb large amounts of heat for a time. The oceans, therefore, act as a break on the climate system retarding change, delaying any warming and cooling. To make things worse, the regional effect of a climate change is not intuitive. In high latitudes, the increased water vapor might fall as snow leading to major surprises as increased fresh water run off halts or slows the so-called thermohaline circulation (driving the Gulf Stream) which plunges far northern surface waters to the abyssal depths returning them to the southern hemisphere. Interrupting this current might plunge the North Atlantic into a deep freeze.

Sooner or later, yes. Because CO2 stays in the atmosphere for about a century, simply freezing the annual emissions won't stop the atmospheric concentrations rising. To stabilize the atmospheric levels of CO2, be it at double, triple, quadruple preindustrial levels (or more), will eventually involve phasing out fossil fuels entirely. This is extraordinarily difficult.Fossil fuels are the basis of modern urban civilization. This highly concentrated form of energy which can be used for electricity, heating and transportation has enabled the evolution of large modern cities and a modern infrastructure to ship people and goods all over the planet. Today, fossil fuels are the primary source of 87% of the world's energy; the remainder coming from biomass (firewood and charcoal), nuclear and hydoelectric. Fossil fuels are not only concentrated and convenient, they are remarkably cheap.

Perhaps the biggest challenge is to cut our dependence at a time when the world's population and energy demands are both rising rapidly. Over the next century, world population will almost double to over 10 billion at least. And the standard of living of billions of people will rise as they seek many of the material benefits of the developed nations. This will require lots of energy; rising from today's energy consumption rate of 10 terawatts (trillion watts) to a rate of over 20 TW by the middle of the century and 40 TW or more by 2100.

Just a little. Switching from high carbon sources like coal to lower carbon sources like natural gas will also certainly buy the world time. The good news is that nations like the U.S. are using energy more efficiently. In a typical American house, the appliances are much more efficient than they were two decades ago: the toasters, TVs, cookers and boilers. The bad news is there are many more appliances like VCRs, computers, video games (many of which are left on); the homes are larger; and the number of automobiles we drive has increased.

There are forms of energy which produce little or no carbon. However, currently, there is no known source (with the possible exception of nuclear energy capable of producing the massive amounts of energy the world will need. Hydroelectric power is essentially carbon free, but the available rivers have all been exploited and hydro will never contribute more a few percent of the total. Nuclear energy is well established--440 plants produce 17% of the world's electricity with no carbon dioxide or other air pollutants. But public concerns about safety issues currently prevent a major expansion in this source.So-called "new renewables" are continually mentioned as possible solutions, but it's difficult to see how sources like solar, wind and biomass can ever play a major role. Solar and wind are intermittent sources (the wind only blows 25% of the time) which gather very small amounts of energy per hectare. Biomass, essentially the construction of energy plantations, which grow cellulose for burning or conversion into ethanol, is even less concentrated. To produce 10 trillion watts (the current world power consumption) using biomass would mean using 10% of the world's land surface area (the area currently given over to agriculture.)