Crops were ruined and people starved. It is useful to recall that we have been measuring temperature with thermometers only for about 150 years. Until recently, surface temperatures were recorded primarily at land-based sites, often close to warmer urban areas, and from relatively crude measurements at sea. Today, we also use satellites to record temperatures around the globe from various heights. Measurements over the past 30 years indicate that atmospheric and surface temperatures are the same.
Scientists use computer models to make their best guesses about future changes in climate.
They consider a variety of factors, including greenhouse gas levels. The forecasts are tested against already known climate patterns and events. So far, the models have failed to agree with historical climate trends. The problem is that computer models cannot yet reliably factor in everything that affects climate. Also, computers are not yet powerful enough to adequately account for some of the major uncertainties of the climate debate, such as the hydrological cycle and clouds.