Large changes in tropical rainfall expected due to greenhouse gas emissions

29 09 2015

A new Met Office study has found that, if global greenhouse gas emissions are not reduced, we are likely to see large changes to the rainfall in tropical countries.

Scientists found that the size of these changes will be strongly determined by the total amount of greenhouse gases that are emitted.

Many people in tropical regions, which contain some of the least developed countries in the world, are already exceptionally vulnerable to variations in how much and how frequently it rains.  Any large, long-term changes to rainfall amounts due to climate change could worsen this vulnerability, and test the ability of societies and wildlife to adapt to potentially unprecedented conditions.

Climate simulations of tropical land rainfall change and global temperature change over the 21st century under four different greenhouse gas emissions scenarios.

Climate simulations of tropical land rainfall change and global temperature change over the 21st century under four different greenhouse gas emissions scenarios.


To investigate possible changes in future rainfall patterns, Met Office scientists used a large number of climate change simulations of the 21st century, produced by research institutes across the world.  All simulations run with high greenhouse gas emissions produced large changes in rainfall patterns across substantial areas of tropical land by the end of the century.  On average around one quarter of all tropical land was affected – an area twice the size of Brazil. Simulations run with lower greenhouse gas emissions showed much smaller areas of land with large rainfall changes.

Which regions are most vulnerable to rainfall change?

Analysis of future climate simulations shows that under high greenhouse gas emissions, large rainfall changes are expected to occur over an even larger area of dry land than was affected during the Sahel drought. Of course the impacts of such changes would be heavily dependent on the resilience of the particular countries affected.

The long-term drought in the Sahel region of West Africa brought famine to hundreds of thousands of people and huge disruption to millions more in the 1970s and 80s. Although not clearly linked to greenhouse gas emissions, the Sahel drought provides a yardstick for the potential impacts of future climate change.

When and why will these changes happen?

Rainfall changes in tropical countries are expected by the end of the 21st century due to climate change affecting a number of the processes which determine where and how much it rains in different parts of the tropics. One of these is the pattern of surface temperatures across the tropical oceans. As rainfall tends to occur over the warmest parts of the oceans, any changes to these patterns can cause large changes to the regions where it rains – this is what happens during an El Niño event.

Exactly where will changes occur?

Exactly which countries will be affected by these future rainfall changes is much less certain, as climate simulations disagree on where the changes will occur. Regions thought to be most at risk of large decreases include southern Africa and Central America, while India and East Africa are among those most likely to experience large increases. However it should be emphasised that the location of changes is much less consistent among climate simulations than the fact that large changes occur.

Stark weather contrasts across the USA

18 02 2015

While the UK continues to see fairly typical winter weather, over the other side of the Atlantic the US is experiencing some stark contrasts.

While some parts in the west are seeing warm and dry conditions, eastern areas are seeing very cold weather.

This week will see a continuation of warmer-than-average conditions in western parts of the USA, with little or no rainfall in the forecast.

Map showing air temperatures across the US, with white (-24C) and blue (below 0C) showing cold air and yellows and oranges showing warm air. From the Met Office's Global Model for 1200HRS GMT on 20 February 2015

Map showing air temperatures across the US, with white (-24C) and blue (below 0C) showing cold air and yellows and oranges showing warm air. From the Met Office’s Global Model for 1200HRS GMT on 20 February 2015

Despite some welcome rainfall at the start of February, California remains in drought, with a large swathe in exceptional drought – which is the highest category that the US Drought Monitor report.

In San Francisco, no rain fell at the downtown observation station or the airport during the whole of January 2015. This is the first January without rainfall since records began in 1850. Normally January is the wettest month of the year, with an average 119mm.

The dry conditions have also resulted in the Sierra Nevada snow pack being at less than 50% of where it should be as we head towards the end of winter.

Meanwhile, the very cold spell of weather is expected to continue across a large part of eastern and northeastern USA, with air originating from the Arctic keeping things icy.

There will be some snow at times, although not as significant as some recent events, though localised heavy ‘lake effect’ snow is likely this week off the Great Lakes.

However, the most noteworthy element will be the extreme cold. Another arctic front will arrive across the East Coast, bringing exceptionally cold conditions.

Places from the Carolinas to the Mid-Atlantic may see some of the coldest weather since the mid-1990s, with numerous record low temperatures expected.

In fact this cold air is expected to reach as far south as Florida, with even the Caribbean expecting well below average temperatures throughout the rest of this week.

Guest blog: ‘Risk of summer drought at normal levels’

17 06 2014

There have been some reports in the press that the Met Office has warned dry weather this June could bring a return of drought conditions to the UK – this is not the case. Here Victoria Williams, Water Resources Advisor at the Environment Agency, explains what the real risks are at the moment:

Every week we measure water resources in England to assess how dry the soils are and how much rain they can soak up, the amount of water flowing in rivers, stored below ground in aquifers and above ground in reservoirs, and the outlook for the coming months.

As we move into summer the overall water resources situation across England is looking generally healthy. This is not surprising given England has experienced the wettest six month period (Dec-May) on record.

Regionally it has also been a record breaker with the wettest six months experienced in southeast and southwest England and the second wettest in central and northwest England.

All our rivers have responded to the rainfall and are currently within normal ranges.  Groundwater levels throughout England are within normal ranges and are now starting to recede as expected for the time of year.

We also look ahead by modelling how rivers and groundwaters may respond to different future rainfall patterns over the summer. The results shows a broadly positive picture even if rainfall is below average and point to the risk of drought this summer being no greater than average.

However it is still as important as ever to use water wisely. If the weather does turn hot and dry there can be localised impacts on rivers, the environment and for farming. If this happens we work with abstractors to reduce the effects where possible and water companies will keep their customers informed if needed.

For more information see the Environment Agency water situation reports.

Why does it always rain on the UK?

9 05 2012

After the wettest April in records dating back to 1910 and an unsettled start to May, parts of the UK are set to see more heavy rain today and tomorrow.

With all the wet weather, many people have been asking what is to blame and whether something unusual is going on.

In an earlier article on this blog we looked at how the jet stream has influenced the recent spell of unsettled weather, but stressed it is not the only factor at play.

While the jet stream may be an influence, there is nothing unusual about its current position and it regularly behaves in this way.

With that in mind, it’s possible to go a step further and say there is nothing unusual about the UK’s weather over the last few weeks.

That may sound odd on the back of a record-breaking wet month, but we do expect to see records broken and they do topple fairly regularly for one area or another.

The past April fits into this expectation – it was exceptionally wet, but only slightly wetter than the previous record set just a few years ago in 2000 and there are several years close behind.

We only have to look back another month to see that March was the joint warmest on record for Scotland. Looking further back, parts of the UK have seen some of their driest months on record in the last year or so, and we saw the coldest UK December on record in 2010.

The mixture of record-breaking months in recent history illustrates what’s called natural variability – which is a way of summing up the inherent random or chaotic nature of weather. This is why our weather is different from one week, month or year to the next.

Here in the UK that variability is particularly noticeable because of our location. We sit in the mid-latitudes where cold air from the poles meets warm air from the tropics, and have the Atlantic on one side and the large landmass of continental Europe on the other.

All these factors mean our weather can be highly variable and we can see periods of unsettled, wet and windy weather at any time of year – a challenge that the Met Office has to rise to every day to provide the accurate weather forecasts that you, businesses and our government partners have come to expect.

April already wettest in a century

30 04 2012

This April is the wettest in the UK  in records which date back to 1910, according to early Met Office figures up to the 29th of the month.

Further rain is to come overnight tonight as outbreaks of heavy and possibly thundery rain affect southern England and Wales, so final rainfall figures for the month will change.

However, the UK has already seen 121.8mm of rain so far – significantly more than the 69.6mm you would normally expect for the month and beating the previous record of 120.3mm set in 2000.

Read more on the Met Office website.

Wet and windy weekend

29 04 2012

Large parts of the UK have seen very wet and windy conditions over the weekend.

Winds have gusted up to about 70mph in the most exposed locations, with many parts of the country seeing gusts of 40-50mph.

The strongest gust (at a non-mountain site) was 71mph at Mumbles Head in West Glamorgan, Berry Head in Devon was just behind with 70mph.

Leek, Thorncliffe in Staffordshire saw 68mph, Avonmouth in Avon saw 61mph, and two locations on the Isle of Wight also saw 60mph.

Persistent and heavy rainfall has also been widespread across a large part of the country, with some areas seeing a significant proportion of their normal monthly average within a 24-hour period.

The heaviest rainfall has been focused on southern England with Liscombe in Somerset seeing the most rainfall so far, with 48.8mm falling between 1pm on Saturday, 28 April and 6pm today, 29 April.

Hampstead in Greater London saw 39.2mm of rain during the same period, with Wiggonholt in West Sussex seeing 32.4mm and Rothamstead in Hertfordshire seeing 29.8mm. Several other stations across the south saw rainfall at a similar level.

Other parts of the country, particularly in the far north, have seen very little or no rain at all, however.

Looking ahead, there are Severe Weather Warnings in place for both Monday and Tuesday in parts of the UK, so we’d advise people to stay up to date with the latest forecasts and warnings on our website.

The Environment Agency has also issued many Flood Warnings and Alerts – you can stay up to date with the latest information via their website.

Guest blog: It’s raining – why have we still got a drought?

20 04 2012

Trevor Bishop is Head of Water Resources at the Environment Agency. Here he explains why, despite the rain, we are still in a drought.

In true bank holiday spirit the weather turned wet for the Easter holidays and it doesn’t seem to have stopped since. So is there still a drought? The answer is yes – it’s going to take more than a week or two of rain to undo the effects of nearly two years of below average rainfall. The recent rain is good for farmers and gardeners, and the cool temperatures ease the pressure on fish and wildlife in rivers. But with dry soils most of the rain will be soaked up – or, worse still, run off quickly if the surface is compacted, causing flash floods. But it won’t reach down far enough to top up groundwater, which is what we really need.

More rain now will really help us get through the summer, and is good for the environment, farmers and gardeners, but it’s very unlikely to be enough to recharge the groundwater. As we move from spring to summer, most of the rain that falls is either evaporated as temperatures rise or taken up by plants as they grow.

You can keep up to date with the water situation here.  This shows that while river flows have recently risen in western and northern England and in Wales, groundwater and rivers in the south and east remain exceptionally low for this time of year. Much of our tap water in the south east comes from groundwater so it’s still important we use less water, even when it’s raining.

Met Office in the Media: 19 April 2012

19 04 2012

The wet weather across parts of the UK at the moment has generated comments about how wet this April as a whole may pan out in the record books.  As we have said in blogs previously, it is just too early when we are still only mid month to make any assessment about how the month as a whole may compare to previous months in history.

Today the Daily Telegraph reported that the ‘the wet weather will continue to the end of this month, including floods and storms, according to the Met Office, potentially making it one of the wettest April’s on record.’  Although our outlook does suggest that the rather unsettled conditions are likely to continue through the rest of the month, we have not made any assessment on whether it is likely to be one of the wettest April’s on record.

Looking at the latest figures available, the UK has seen around 60% of its normal April rainfall, or 41.7mm up to the 15th of April.  At this point of the month we would normally expect about 50% of the months rainfall so rainfall amounts so far have not been far from what you could typically expect. On average, a typical April would see 69.6 mm or rain.

The Daily Express has today run with a headline that we are expecting the ‘Coldest May for 100 Years’.  This forecast has not come from the Met Office, but from an independent forecaster. Currently our 16 to 30 day forecast which takes in the first half of May says:

“The start of May looks likely to remain unsettled with a continuation of showers or longer spells of rain, although there should also be some drier and brighter interludes. Temperatures will generally be close to or slightly above the seasonal average. Later in the first week of May, conditions may turn more settled across southern England for a time, with a greater chance of some drier and sunnier weather than of late. Further north, it looks likely to stay unsettled with further rain at times, particularly across northwest England as well as northern and western parts of Scotland.”

Why are we in drought?

13 03 2012

Rainfall amounts across many parts of the UK have been below average for the last two years. Importantly, this includes two dry winters – the periods when we would normally expect our rainfall to replenish river, reservoir and groundwater levels.

2010 was the eleventh-driest year in the series from 1910 and the driest since 2003. The dry weather continued during 2011 with large parts of central, eastern and southern England having well below average rainfall – several Midland counties – such as Shropshire, Nottinghamshire, Leicestershire and Warwickshire – had their driest year on record.

Map showing the rainfall seen between Nov 2010 and Feb 2012 compared to the long-term average for the same period

This emphasises why there are concerns about drought in parts of England and Wales.

There is no one reason for the dry weather over the last few years; it’s all part of the natural variability of the UK climate. However the dominance of high pressure systems over Europe has been a significant influence. These areas of high pressure effectively block the path of the Atlantic weather systems that bring us our usual wind and rain and this has happened relatively frequently over the last couple of years.

Looking back to the drought of 1976 we can see that in contrast to this year, the whole of the UK was subject to a prolonged period of below average rainfall. Between May 1975 and Aug 1976 only parts of the Western Isles and Western Highlands saw rainfall even a little above normal. Many parts of England and Wales saw only half or less of the rainfall that we could normally expect in that time.

Map showing the rainfall seen between May 1975 and Aug 1976 compared to the long-term average for the same period.

This table shows how much rainfall fell across the UK in 1975-76 compared to 2010-12:

Region/district May 75-Aug 76 total (mm) % of 61-90 average Nov 10-Feb 12 total (mm) % of 61-90 average
UK 1018 72 1502 99
England 689 64 929 82
Wales 1097 62 1658 84
Scotland 1527 82 2386 117
Northern Ireland 1098 78 1638 108
England & Wales 745 64 1030 82
N England 855 69 1212 94
S England 601 60 780 74
N Scotland 1742 86 2564 114
E Scotland 1062 74 1730 113
W Scotland 1734 83 2843 123
E & NE England 666 67 901 88
NW England & N Wales 1160 71 1690 96
Midlands 622 61 780 74
East Anglia 524 65 579 72
SW England & S Wales 907 59 1383 81
Central S & SE England 584 59 824 78

We are working closely with the Environment Agency, Government, local authorities and water companies to ensure that the UK best manages the need for water, while protecting agriculture, horticulture and the environment.

Our role includes providing forecast information to the public; government and the water companies on when, and how much rain, is expected from the next few days to a month ahead. We also maintain observations of how much rainfall there has been and where. This can be compared with our historical database of rainfall, which goes back to 1910, to provide an understanding of the current rainfall situation. The Met Office also provides the UK’s only real-time assessments of rainfall, evaporation and soil moisture for water resource specialists.

Rob Varley, Operations Director at the Met Office, said: “The last two years have been very dry across many parts of England with some areas seeing as little as 60% of their normal rainfall in that time.

“Even sustained rainfall over the next few months would have a limited impact, however we are working with the water industry to make sure they have the best weather forecast information available to help them manage their resources.”

Howard Davidson, Environment Agency South East Regional Director said: “We will continue to work with water companies to meet the challenges of a continued drought.

“The Environment Agency’s role is to balance the water needs of people, businesses and the environment. Using water efficiently will help to ensure we all have enough water for our homes, to produce food, products and services, and to protect our valuable natural environment and wildlife.”

Related articles:

A tale of two halves for February and winter

28 02 2012

As February and winter draw to a close, early statistics show that both have been stories of two halves.

We often talk about why you can’t pre-judge a month or a season at its half-way stage, and the latest figures perfectly illustrate why that’s the case.

The mean temperature for the first half of February was very low as cold weather gripped the UK – particularly in England where temperatures were 4 °C colder than the long term (1971-2000) average.

If you’d projected those figures out to the end of the month you would have expected one of the coldest Februarys on record.

However, the second half of the month (figures go up 26 February) has seen some exceptionally mild weather to balance things out and we have ended up with a rather average month for overall UK mean temperature which so far, was just 0.1 °C above average.

This story of two halves can be seen in the maps below, with the blue colours denoting lower than average temperatures in the half-month figures on the left, and the more balanced situation by the end of the month on the right.

Winter, which meteorologically speaking runs from December to February, has been a fairly similar story – but in reverse.

A mild December and first half of January meant we had a very mild first half of the season, which led to some media headlines mooting one of the mildest winters on record.

However, the last few days of January and the first half of February were colder than average, bringing the overall temperature for the season down.

With a couple of days still to go, the early statistics show the UK’s mean temperature for winter is 0.7 C above average, making this a mild winter – comparable with several other mild winters in the last decade.

One common theme between this February and winter as a whole is dry weather – particularly for the south and east of the UK.

February was particularly dry, with the UK having seen just 62% of the normal amount of rainfall we’d expect for the whole month by the 26th. With a few days left, this is unlikely to change by much.

England was the driest country, having so far seen just 43% of the rainfall we’d expect for the month and Wales not far behind at 49%.

Winter has also been dry overall in England, with just 82% of the rainfall expected for the season and Wales has seen 89%. This is slightly balanced out in the statistics by Scotland being wetter than normal over the season, seeing 116% of its normal rainfall.

The relative lack of rainfall for February and the season as a whole can be seen in the two maps below, with the brown colours denoting drier than average weather for the month on the left and for the season on the right.


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