Climate data come from many sources, and each has limitations that affect how reliable it is.Temperature records: early thermometers were less precise, and weather stations were concentrated in Europe and North America, with few over the oceans, polar regions or Africa. Stations may have moved, or towns may have grown around them, causing the urban heat island effect, where a built-up area is warmer than the countryside. Scientists correct for these, but corrections add uncertainty. Satellites now give global coverage.Dendrochronology: ring width is affected by rainfall, soil, disease and competition as well as temperature, so it is not a pure temperature record. It only works where trees grow.Pollen: pollen may be blown or washed in from elsewhere; some plants produce more pollen, or pollen that preserves better, than others; and plants take time to colonise new areas after the climate changes, so the pollen record lags behind temperature.Ice cores: deep layers are harder to date precisely, and the record comes from polar regions only.To judge reliability, ask whether:
• the data are precise (repeat readings close together) and accurate (close to the true value);
• enough sites and a long enough time were sampled;
• independent methods agree.Although each method has weaknesses, confidence that the Earth is warming is high because different, independent methods show the same trend. When several lines of evidence agree, together they are far more reliable than any one of them alone.
• the data are precise (repeat readings close together) and accurate (close to the true value);
• enough sites and a long enough time were sampled;
• independent methods agree.Although each method has weaknesses, confidence that the Earth is warming is high because different, independent methods show the same trend. When several lines of evidence agree, together they are far more reliable than any one of them alone.