The global average temperature has increased over the past 160 years, but short-term trends in temperature and sea ice seem to be at odds with each other and need more research, according to UK scientists.
In a report on long and short-term climate trends, the UK Met Office's Hadley Center found several factors that indicate a warming world and say 2010 has been one of the warmest years on record.
The report drew on the work of more than 20 institutions worldwide and used a range of measurements from satellites, weather balloons, weather stations, ocean buoys, ships and field surveys.
The report showed increases in air temperatures above both land and sea, increases in water temperature and humidity, sea level rises and the shrinking of Arctic sea ice.
"The average temperature over the first decade of the 21st century was significantly warmer than any preceding decade in the instrumental record, stretching back over 160 years," the report says.
Despite variability from year to year, with some years warmer and others cooler, a clear trend of increasing global temperature can be seen from the late 1970s onwards at about 0.16°C per decade, the report says.
"It is clear from the observational evidence across a wide range of indicators that the world is warming," says Dr Matt Palmer, ocean observations specialist at the Met Office.
"As well as a clear increase in air temperature observed above both the land and sea, we see observations which are all consistent with increasing greenhouse gases," he added.
Slow down in warming
But, short-term trends in temperature and sea ice seem to be at odds with each other. The rate of temperature increases has slowed over the past 10 years, while the level of sea ice has increased.
Climate models suggest that the internal variability of the climate system may be responsible for the recent decrease in the rate of warming, the report says.
It suggests changes in solar activity, water vapour, increased aerosol emissions from Asia and changes to the way sea surface temperatures are measured over the past decade could have contributed to some artificial cooling.
"We expect warming to increase in the next few years ... However, other future external factors, such as volcanic eruptions or changes in solar activity, could prolong the current reduction in warming," the report says.
More research is needed into some of the factors that influence short-term climate trends, which are not fully understood or represented in climate models.