The observed increase in global surface temperatures is unequivocal and a clear manifestation of global warming.

That conclusion comes in particular from 150 years of data collected by the 188 members of the World Meteorological Organisation through observing networks of tens of thousands of stations on land, at sea, in the air and from constellations of weather and climate satellites.

Several types of evidence support this conclusion, and the warming has further accelerated since the middle of the 1970s, particularly in the past 20 years. In fact, since the beginning of the 20th century, the global average surface temperature has risen 0.74C. The 11 warmest years on record occurred in the past 13 years.

Evidence of global warming has also been documented in widespread decreases in snow cover, sea ice and glaciers.

Scientists with the International Polar Year report that during the summers of 2007 and 2008, the minimum extent of year-round Arctic sea ice decreased to its lowest level since satellite records began 30 years ago. The increased melted land ice and snow, coupled with sea waters expanding due to warmer temperatures, have caused sea levels to rise about 200mm higher than in 1870.

The global combined sea-surface and land-surface air temperature for 2008 was 0.31C above the 1961-1990 annual average of 14.0C. The global average temperature in 2008 was slightly lower than that for the previous years of the 21st century, partially due to a La Nina event (manifesting itself through a cooling of the waters in the eastern equatorial Pacific) that developed in the second half of 2007. Although 2008 was slightly cooler than 2007, it nonetheless ranked as the 10th warmest year on record.

The slight dip from 2007 to 2008 is not a departure from the long-term global warming trend. It is, rather, an illustration of shorter-term variability in the climate.

Variations have occurred throughout the temperature record - spikes and dips that overlay the longer upward warming trend.

Shorter-term variations in temperatures do not dampen the overwhelming long-term increase in global surface temperatures observed since 1850, when reliable meteorological records began. The trend remains one of warming.

The Earth's surface temperature has increased by three-quarters of a degree since the middle of the 19th century. This increase, however, is not distributed evenly.

On a yearly basis, we may observe in some parts of the world colder or warmer episodes than in other parts, leading to record cold and hot temperatures. This regional climate variability again does not disprove the long-term climate change.

Weather conditions are the result of extremely complex interactions between land, ocean and atmosphere, and much time is needed before any short-term or small-scale climate variation can be put into the context of longer-term climate change.

Just as one cold snap does not change the global warming trend, one heatwave does not reinforce it.

Still, the Intergovernmental Panel on Climate Change, co-sponsored by the meteorological organisation, has confirmed that based on observations and increasingly sophisticated and realistic numerical models, regional variability has increased and will continue to increase as global surface temperatures rise.

This will likely result in more weather and climate extremes, such as droughts, floods, storms and heatwaves. Responding to this rising challenge requires the collaborative efforts of all countries and of scientists in multiple disciplines to develop adaptation strategies to reduce risk of disaster. This response will be discussed at World Climate Conference-3 in Geneva, Switzerland, from August 31 to September 4.

Using short-term climate variability to argue about global warming and its effects is scientifically inaccurate and a misinterpretation of the data and scientific knowledge.

In addition to climate data analysis and simulation by climate models run on the most powerful computers, scientists have documented global warming based on multiple additional lines of evidence - from ice cores and satellite observations to pollen and sediment records.

Our organisation and its partners are committed to providing the most accurate climate information to decision-makers to help societies address the challenge of climate variability and change worldwide.

* Michel Jarraud, Secretary-General of the World Meteorological Organisation in Geneva, responds to a recent article by Chris de Freitas questioning global warming.