Forecasts for the twenty-first century

The computer models that project global climate don't yet have the skill to issue regional outlooks with much confidence. This means that policy-makers don't work with forecasts per se. Instead, they evaluate 'scenarios”, examples of what could happen if things played out in a certain fashion. Researchers spin whole sets of scenarios under various assumptions: emissions will hold steady, they'll rise, they'll drop, etc. It's then up to policy-makers to decide which scenarios to plan for. Think of this as insurance: people often plan for potential calamities, like a car wreck or a home fire, that they don't necessarily expect will happen.

Research bodies around the world have taken a cue from the Intergovernmental Panel on Climate Change, or IPCC, and produced reports that bring climate change into perspective for a single country or region. Below are summaries of two such studies.

• United Kingdom. The Hadley Centre, part of the UK Meteorological Office, produced the first comprehensive look at British climate-change scenarios in 1998. The 2002 update was dubbed UKCIP02, and a third update (UKCIPnext) is planned for later in the decade.The UKCIP02 scenarios are based on four IPCC projections of how global emissions might increase in the twenty-first century. Overall, the UK is projected to warm by the 2080s by an amount that ranges from 2.0°C/3.6°F in the lowest-emissions scenario to 35°C/63°F in the highest The warming is somewhat greater in the summer and autumn than in winter and spring; in all scenarios the southeast warms more than the northwest. In general, winters moisten and summers dry out, with a slight overall drop in precipitation. Snowfall could decrease by anywhere from 30 to 90 percent by the 2080s, depending on the scenario.
  As with their IPCC sources, none of the four UKCIP02 scenarios are judged as to their relative likelihood, because each depends on long-term societal choices that are virtually impossible to predict For UKCIPnext, the probability of particular outcomes within each scenario will be assessed through ensemble modelling. This involves producing a number of model runs for each scenario, with tiny changes to the model starting points that capture a wide range of future climates and gauge how likely each might be.
• United States. The first US National Assessment, mandated by Congress and released in late 2000, was the result of nearly a decade of work. The assessment is still available on the Web; despite its age, it remains one of the most detailed looks at how US climate might change with global warming. Instead of looking at a range of scenarios from one model, the US assessment used one scenario each from two global climate models, one from the Hadley Centre and the other from Canada. The emissions levels in both are the same as the medium-high UK projection (increases of about 1 percent per year). The average US temperature is projected to rise a slight bit more rapidly than the rest of the world's in the twenty-first century. An average of 0.28°C/0.5°F per decade is predicted by the Hadley model and 0.5°C/0.9°F by the Canadian model. As in the British projections, the US warming is most prominent during winter but present year round. In the already-steamy Soutfv the average July heat index (a measure of heat and humidity combined; see Resources, p.402) is projected in the Canadian model to climb more than 11 °C/20eF by 2100. Both models project big rises in precipitation for the southwestern deserts, with increases topping 80 percent in some spots.The models disagree on the eastern US: Hadley projects widespread increases in the 20-30 percent range, while the Canadian model dries out much of the southern tier by 10 to 30 percent. More recent models still disagree on how US precipitation might change, but in any event it's believed that the warmer US temperatures are liable to stimulate more intense and/or prolonged droughts. The Atlantic and Gulf coasts will be vulnerable to rising sea levels and storm surges that could jeopardize wetlands, while the levels of the Great Lakes are projected by some models to decline.
  Succeeding the US National Assessment is the US Climate Change Science Program, which is centered on a series of 21 'synthesis and assessment” reports that will characterize the state of global-change research in various areas over the next few years. These reports are intended to clarify key questions about climate-change science and lay the groundwork for future research; the/re not designed to provide specific regional projections in the style of the US National Assessment.