Nevertheless, detecting a significant rise in global average temperature over the past 100 years would be strong evidence that the climate is indeed changing as the models predict it should. Global temperature records exist only for the past century or so. All begin around 1860. Before then, data collection was not systematic enough for a global record. Land-based records come mainly from meteorological stations.
Marine records rely principally on observations made by merchant ships.
In both cases, therefore, more observations have always been made in some regions, such as densely populated areas or well-travelled shipping lanes, than in others. All data must therefore be analysed carefully to ensure that such patchy coverage does not introduce systematic biases. Before then, however, even careful analysis may lead to long-term averages that are either too warm or too cold by up to 0.1 C.1 Records may be affected by changes in the way observations are made. Observation methods, such as the time of day that air temperatures are taken, affect what is recorded. If methods were applied consistently throughout an observation period, this would not matter.