Over the past 100 years the global mean surface temperature has increased by about 1 °C. This warming trend followed after the so-called Little Ice Age (~1600 to ~1850), a cooler period that occurred over a large part of the globe with the strongest signal at the mid- and high latitudes of the Northern Hemisphere. During the Little Ice Age, glaciers reached maximum stands in many parts of the world and sea-ice extent in the Arctic seas was significantly larger than today.
The most important factors that cause temperature fluctuations on a decadal-to-century time scale are: (i) variations in solar activity, (ii) aerosol loading of the higher atmosphere due to explosive volcanic eruptions, (iii) long-term shifts in ocean circulation and associated heat transports, and, last but not least, (iv) changes in the composition of the atmosphere (most importantly CO2) due to human activities. Researchers have used climate model simulations to disentangle the various factors (e.g. Lehner et al., 2013).

There is little doubt that a substantial part of the warming over the past 50 years is a direct consequence of anthropogenic greenhouse gas emissions (IPCC, 2013). The significant warming seen in the first part of the 19th century may be partly due to a smaller number of volcanic eruptions in that period.

Warming is not uniform over the globe. The strongest signal is seen in the high latitudes of the Northern Hemisphere (the 'polar amplification'), probably related to cryospheric feedbacks (sea ice, snow cover) on the surface energy balance. Regional differences can also be large. The temperature increase in central Europe, for instance, is more than twice the global mean value. Such differences are often related to shifts in the major weather systems like the quasi-permanent subtropical high pressure cells (e.g. the Azores High).

Abbildung erstellt von NOAA (climate.gov)

References

IPCC (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker TF, and 9 others (eds)]. Cambridge University Press, 1535 pp, (doi:10.1017/ CBO9781107415324)

Lehner F, Born A, Raible CC and Stocker TF (2013): Amplified inception of European Little Ice Age by sea ice-ocean-atmosphere feedbacks. J. Climate, 26, 7568-7602