Key findings: Observed trends

Reposted from:http://ukclimateprojections.defra.gov.uk/content/view/512/9/index.html

UKCP09 science report: The climate of the United Kingdom and recent trends

Average global temperature and sea level have risen since the late 19th century, and at an increased rate over the past few decades.

• Warming of the global climate system is unequivocal, with global average temperatures having risen by nearly 0.8 ºC since the late 19th century, and rising at about 0.2 ºC/decade over the past 25 years.

• It is very likely (>90% probability, IPCC) that man-made greenhouse gas emissions caused most of the observed temperature rise since the mid 20th century.

• Global sea-level rise has accelerated between mid-19th century and mid-20th century, and is now about 3mm per year. It is likely (>90% probability, IPCC) that human activities have contributed between a quarter and a half of the rise in the last half of the 20th century.

Average UK temperature has risen since the mid 20th century, as have average sea level and sea surface temperature around the UK coast. Over the same time period, trends in precipitation and storminess are harder to identify.

• Central England Temperature has risen by about a degree Celsius since the 1970s, with 2006 being the warmest on record. It is likely (>66% probability, IPCC) that there has been a significant influence from human activity on the recent warming.

• Temperatures in Scotland and Northern Ireland have risen by about 0.8 ºC since about 1980, but this rise has not been attributed to specific causes.

• Annual mean precipitation over England and Wales has not changed significantly since records began in 1766. Seasonal rainfall is highly variable, but appears to have decreased in summer and increased in winter, although with little change in the latter over the last 50 years.

• All regions of the UK have experienced an increase over the past 45 years in the contribution to winter rainfall from heavy precipitation events; in summer all regions except NE England and N Scotland show decreases.

• There has been considerable variability in the North Atlantic Oscillation, but with no significant trend over the past few decades.

• Severe windstorms around the UK have become more frequent in the past few decades, though not above that seen in the 1920s.

• Sea-surface temperatures around the UK coast have risen over the past three decades by about 0.7 ºC.

• Sea level around the UK rose by about 1mm/yr in the 20th century, corrected for land movement. The rate for the 1990s and 2000s has been higher than this.

Average temperature across all regions of the UK has risen since the mid 20th century. Over the same time period, trends in precipitation are harder to identify.

• All regions of the UK have experienced an increase in average temperatures between 1961 and 2006 annually and for all seasons. Increases in annual average temperature are typically between 1.0 and 1.7 °C, tending to be largest in the south and east of England and smallest in Scotland.

• The annual number of days with air frost has reduced in all regions of the UK between 1961 and 2006. There are now typically between 20 and 30 fewer days of air frost per year, compared to the 1960s, with the largest reductions in northern England and Scotland.

• There has been a decrease in the average number of Heating Degree Days (HDD), and an increase in the average number of Cooling Degree Days (CDD) in all adminstrative regions of the UK as a whole, between 1961 and 2006.

• There has been a slight increase in average annual precipitation in all regions of the UK between 1961 and 2006, however this trend is only statistically significant above background natural variation in Scotland where an increase of around 20% has been observed.

• There has been an increase in average winter precipitation in all regions of the UK between 1961 and 2006, however this trend is only statistically significant above background natural variation in Northern England and Scotland where increases of 30 to 65% have been experienced.

• There has been a slight decrease in average summer precipitation in most regions of the UK between 1961 and 2006, however this trend is not statistically significant above background natural variation.

• Average annual and seasonal relative humidity has decreased in all regions of the UK, except Northern Ireland, between 1961 and 2006, by up to 5%.

• There are no statistically significant trends in the average number of rain days or mean sea level air pressure for any region of the UK between 1961 and 2006.