Climate Change: Fact or Fiction ?
Prof. Dr. Mojib Latif (Ozeanzirkulation und Klimadynamik, Helmholtz-Zentrum für Ozeanforschung (GEOMAR), Kiel)

Given all the warnings about and plans to forestall global warming, people may be surprised to find, over the next several years, that summers are no warmer than before, maybe even a bit cooler—and that winters are as cold, or colder, than they have been in the past couple of decades. This is because the climate may go through a period of temporary cooling; it’s nothing unusual, just a natural fluctuation. It doesn’t mean that global warming is not still be at work, or that we no longer need to worry about global temperatures rising by as much as 6°C by the end of the century — an unprecedented warming in the history of mankind if no measures are taken to cut global carbon dioxide emissions. The only problem is that by considering the mean of many models of global warming the natural fluctuations are averaged out, if they were not initialized by the current climate state, and this can be confusing.

The past record of globally averaged surface air temperature illustrates how gradual warming and short-term fluctuations can operate side by side. The temperature ups and downs superimposed on the 20th century warming trend reflect the natural variability. To some extent, we need to ignore these fluctuations, if we want to detect the human influence on climate. Consider, for instance, the early-century warming that was observed from 1920 to 1940. Had forecasters extrapolated into the future, they would have predicted far more warming than actually occurred. Likewise, a brief cooling trend, if used as the basis for a long-range forecast could erroneously support the idea of a rapidly approaching ice age.

Natural climate variations may be produced by the climate system itself. A well-known example is El Niño, a warming of the Equatorial Pacific occurring on average about every 4 years. The record El Niño 1997/1998 helped to make 1998 the warmest year to date. Volcanic eruptions and fluctuations in solar output can also bring temporary climate change. The Philippine volcano Mt. Pinatubo caused a temperature drop in 1992. And an increase of the solar radiation reaching the Earth contributed to the early-century warming. We predict the coming years may see two natural oscillations, the Pacific Decadal and the Atlantic Multidecadal Oscillation, swinging into their negative phase, leading to a cooling of sea surface temperatures in large parts of both the Pacific and Atlantic. A temporary halt in the global warming trend may be the consequence - nothing surprising to a climate scientist.

Mojib Latif is a Professor of Climate Physics and Head of the Ocean Circulation and Climate Dynamics Division at the Helmholtz Centre of Ocean Research (GEOMAR) and member of the Cluster of Excellence “The Future Ocean” of Kiel University, Germany.