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This presentation has evolved from a seminar I gave to the Environment Agency South East Climate Change network in 2013. It was one of a series of seminars that we started running in 2010. I was asked to share the slides online by friends at Climate South East (now path of Climate UK). And March 2016 I was very pleased to be invited to give the presentation again to the Reading Climate Forum.
The aim of the presentation is to help explain the link between Extreme Events, which can dramatically affect our lives, and Climate Change, which may seem less urgent and is often talked about as rather abstract changes in global average temperatures.
In the following slides I present:
- Recap on global average temperature rise
- What extremes lie behind the average?
- A look at extreme events across the world
- ...and events in the UK
- An almanac of the last 20 years
- Questions & Answers
I hope you find these notes useful. If you would like to find out more please e-mail contactchrisbeales.net.
This slide is taken from the 2013/14 IPCC 5th Assessment report. It summarises range of climate change projections from the latest modelling results.
- The black line shows the trend in historic temperatures up to 2012. Since then we have had the two hottest years on record: with 2015 at over 1 degree Centigrade (1ºC) warmer than since the start of the last century!
- The blue bands is what trying to stabilise temperatures to meet the 2ºC target that you will probably have heard about on the news. This is compared with the red band, which is how temperatures are projected to increase if we do not limit our greenhouse gas emissions (e.g. use of coal, oil and gas).
- You can find out more about emission scenarios and their consequences for global temperature change on my website, as well as a discussion about Emissions Targets.
- It is very important to notice however, that all of these temperatures are global annual averages! These trends will 'smooth out' individual extreme events.
At the top are a selection of words that we associate with extreme weather-related events. Notice that we often hear them on the news...
Rather than shifts in average temperatures, it is the extreme events that we worry about. These are the droughts, heat waves, floods, etc that make life difficult or even disastrous.
What's behind the average?
This slide shows global average (surface) temperature maps for September 2015 and October 2015. Notice that each map is divided into squares, for which the monthly average temperature has been calculated.
- The global average temperate for each months will be calculated as the average of all of these squares.
- Notice that the September and October 2015 averages were the warmest on record, and yet there is variation across the maps: some parts of the world were very warm whereas some parts of the world were cooler than normal.
- The global annual average - discussed in slide 2 - will be the average of all 12 months, for all parts of the world. It is worth noting, that as 2015 was the warmest year on record, there were a lot more examples of warm and hot parts of the world than cooler areas.
I've adapted this graphic from IPCC special report on extreme events (page 5, figure SMP3).
- The 'bell-shaped' curve (black line) shows a typical distribution of natural events: they could be temperatures, rainfall, storm surges, etc. We'll use summer maximum temperature in this example, i.e. the hottest day of the summer over the last 30 years.
- The 'x-axis' (left-to-right) gives the range of hottest day temperatures: from coolest on the left, to hottest on the right.
- The 'y-axis' (up) gives an increasing likelihood that the event will occur. Notice that the curve peaks in the middle, showing that most frequently the temperature of the hottest day is close to the average for the last 30 years.
- The extremes are to the far-left and far-right of the graph, and notice that these happen less frequently.
Thinking back to the slide 2, with the increase in average annual temperatures. This will imply a shift in our temperature distribution, which is demonstrated with the dashed lines.
- If this shift in average temperature is accompanied by a shift in the whole 'bell-shaped' curve, notice that this will have a significant effect on the extreme events.
- In this example you can see that there are fewer cool extremes but a lot more hot extreme events.
- You can also see that the hottest events are now even further to the right, implying new record breaking hot temperatures are likely in this warmer climate.
The complete figure from the IPCC special report on extreme events (page 5, figure SMP3) is shown here.
- You can see that there are other possibilities for the shift in the full distribution of temperatures. Charts (b) and (c) give examples of how the shape of the curve might change. This would imply more, fewer or a similar frequency of extreme events.
- Notice though that the average is predicted to shift towards warmer temperatures, with climate change, so curves (b) and (c) still need to be shifted to the right.
- Extreme events are often harder to predict, as they test the limits of our science and modeling. Hence it is very important that we pay attention to any developing trends in our current weather to understand how the distribution is changing.
- We must also consider a precautionary approach to our plans for the future. It is very likely that we will need to adapt to events that are more frequent and more extreme than we have experienced in recorded history.
The US National Oceanic and Atmospheric Administration (NOAA) has a great website with the State of the Global Climate. This is updated regularly and you can view maps and statistics about the climate, annually or for individual months.
- The example shown here is the map of extreme events for the year 2013 [don't worry about the small writing, I will zoom-in on a few parts examples in the following couple of slides].
- The most obvious thing to note is that there are a lot of events marked on this map!
- For reference, statistics for the global average temperature in 2013 are that it "... ties with 2003 as the fourth warmest year globally since records began in 1880. The annual global combined land and ocean surface temperature was 0.62ºC (1.12ºF) above the 20th century average of 13.9ºC (57.0ºF)."
We have zoomed-in on a few examples from 2013 here:
- There are selection from many examples of extreme heat events across the globe, whether they are record-breaking or just very high ranking. Notice that some of these events are short period (e.g. 'the hottest day'); others - like the sea ice - are indicative of many weeks / months of warmth.
- There are more than just temperature events noted. It is worth considering that the atmosphere is a very dynamic system. Heat is not just held as warm air: it leads to faster evaporation of water, which is likely to result in heavy rainfall events (e.g. China and Russia in August) and bigger storms.
- There are also examples of extremes from the other end of the scale (e.g. the cold Spring in England and France).
We have zoomed-in on a few examples from 2013 here:
- The drought in the USA is indicative of many months of shifted weather patterns.
- And we must not forget an example of an extremely high energy system: the devastating Typhoon Haiyan. This was "... one of the most powerful storms ever recorded to hit land, made landfall over the central Philippines, causing catastrophic damage ... Thousands of people were killed and the UN estimates more than 11 million people were affected and some 673,000 displaced..." (Metoffice Barometer website article).
There are extreme events recorded across the world for 2013. This slide shows the events in 2015, which similarly shows a lot of events.
- We would expect to see some happening: this is normal, natural variability in weather patterns. Records are there to be broken ... particularly short ones!
- The authors of the webpage at NOAA will also have searched around for the most 'exciting' events to put onto the map ... that is what it is for.
- However, as I mentioned, there are a lot of events, and most of them are on the hot / high-energy side. This indicates that our 'bell curve' is shifting in the same direction as the global average.
If you think about that global map, compared to the global average temperature graph, it is sobering to wonder about what extreme events we might see in the future. If we follow anything like that red line, the future will be very hard to adapt to!
Here is another global map from the NOAA website. This is for the month of February 2014.
You can see that the UK gets a mention: it says it was '...the forth wettest February since records began in 1910'. This is a reasonably impressive statistic but, having had first-hand experience of this event, I know there is more to that rainfall event...!
The previous slide gave us a statistic for the whole of the UK, and just for the month of February. This has the effect of watering-down the significance of the event, which was actually focused mainly on southern England, and lasted for two-and-a-half months.
- In that time parts of the southeast received over 80% of a whole years' average rainfall.
- For the catchment of the River Thames this broke the record for the biggest two-and-a-half month rainfall event in our records (which go back to 1883).
The consequence of this rainfall was a record-breaking flood on the River Thames, as well for various other rivers in the south of England.
- The graph shown here compares the event to the two greatest flood peaks that we have records for (again since 1883).
- I drew this to show that the volume of water in the 2014 event was actually equivalent to the 1947 and 1894 flood added together!
- Notice though that the 2014 flood peak was not as high, and hence the extent of the flood was much less than those historic peaks. It was still, of course, very distressing for those who did flood.
- The duration of the high flows was the big challenge for us this time around. It put a lot of pressure on resources for Environment Agency staff, local authorities, police, and many others.
Only two years later and we had significant flooding in the North of the country. Storm Desmond was one of a series of Atlantic storms that hit this part of the country. In Honister Pass, in the Lake District, over 340 millimetres of rain was recorded in 24 hours. This was a record-breaking event for the country. Many river flow gauging stations also broke records. And, in Carlisle, a one-in-a-hundred year flood defence was over-topped.
Climate scientists and hydrologists are currently investigating this event.
Going back to the Thames basin though...and winding the clock back two years though we had a very different picture.
This is a graph showing flows in the (same) River Thames at Kingston. The black line are the flows from October 2011 through to early April 2012.
- We had just experienced two, consecutive dry winters, which meant that levels in our rivers and groundwater aquifers were exceptionally low.
- The coloured bands (gray → blue → green → red) show where the flows are usually distributed, according to long term statistical ranking. In an 'average' year, for example, they would be in the green band about 44% of the time.
- During this winter, you can see that the flows were very low.
- I have also drawn faint coloured-lines on the graph, showing the three major droughts of the last century.
- Notice that in March-April 2012 we were facing very serious drought. Catchment conditions put us in a similar situation as 1976: where public water supplies had to be rationed; many people were cut-off and had to get their water from stand-pipes at the ends of their streets!
As a hydrologist, I was rolling up my sleeves and getting ready for what might be the most extreme event of my career...
In this graph I have added the dashed-pink line, which shows the output of a river flow simulation model that I ran. It shows what the flows might have looked like if we had had a relatively dry spring / summer.
- Note that 2013 rainfall is used in simulation, which ranked 43rd driest out of the 131 year record.
- If we had anything like that rainfall, we would have had a serious drought. Notice the flows in June-July particularly: this would be very difficult for water companies to manage!
...but that isn't what happened!
- We had the 2nd wettest April on record, followed shortly after by the wettest June on record (back to 1883).
- The catchment quickly saturated, and river flows and groundwater levels rocketed up.
- With more wet months in late autumn, overall 2012 was the 7th wettest on record, and some rivers were even in flood by the end of the year.
The switch - from drought to flood - in 2012 was remarkable: to a certain degree the two exceptional events 'cancelled each-other out'. It certainly shows how dramatically weather patterns can change. And it begs questions about whether we are likely to see more of this with the warming climate.
The coloured bands, from the hydrograph on the left of this slide, can be looked at in a different way. We normally use them as a background against which to compare recent flows. What I have drawn in the right-hand chart gives seasonal strips showing how much time the flows spent in each of the coloured bands. I have found this to be a useful technique for visually summarising large amounts of data. Here we are using the whole of the Thames Basin as an analogue for how wet or dry each season has been.
- From top to bottom, each bar alternates between the Winter (October-March) and Summer (April-September) seasons.
- In this representation you are looking at the percentage of each season that the flows were in each band.
- In a 'statistically average' year: 5% of the bar would be red, 44% would be green, and 5% would be black.
- The last 20 years were 4% wetter than 1883-2003 long term average for the Thames catchment. Flows at Kingston reflect this, as you can see from the chart, they were 2-3% more often in the exceptionally and notably high bands (black & blue). Hence flood issues have been more dominant than drought in this period.
- Note that you can use this technique analytically to show how the subtle shifts between climatic periods are affecting river flows in the catchment.
Let's get back to temperature records: fortunately for us we have an impressively long record of average Central England Temperatures, which go all the way back to before the Great Fire of London (1659).
- The orange trace is a very simple smoothing line, to help guide the eye through the year-to-year variations in average temperature.
- You can see that there is a fair amount of natural variation between years; there are also some cooler decades, and some warmer ones.
- Follow the 10ºC line with your eye though: notice that the last couple of decades show a trend of warmer temperatures than any other period in the 356 year record.
- And, as you can imagine, there is more to the story if we look behind those annual averages...
This slide is not a complicated bingo card! What I've created here is a grid showing the rank order of each month / year in the Central England Temperature record, for the last 20 years.
- In this ranking: number 1 is the hottest month / year, going down to the coldest (which will be rank 356).
- Each square is also coloured from red (hottest) - through green - to blue (coldest). Notice that there are a lot of orange and red squares here, which highlights how unusually warm this 20 year period is.
- The top and bottom 10 records are also highlighted in bold.
- Notice that we have had 8 out of the 10 hottest years on record in the last 20 years!
- And 6 of the 12 months have had their record-breaking, hottest month within this period.
Putting the two graphics together, gives an interesting representation of the last 20 years in England / southeast England. This is good background to the following almanac of notable and extreme events.
2000/01: Groundwater flood: Between Sept and Apr, only Jan was not in top 6 to 18 wettest month on record
August 2003: 10 day heat wave. Aug & Sep 3rd and 4th driest months
2006: Hottest year on record: also hottest July and Sept
2007: July flood: following the 3rd driest April, May and June were wet, we then had the wettest July on record (with wettest July day on record with 52mm rain on the 20th)
2009-10: Cold winter
2010-11: Second cold winter: 2nd coldest December on record. Also a dry winter
2011: Warm and dry Spring and Autumn: 6th driest March, 4th driest April, hottest April, 2nd hottest November. A rubbish summer though.
2012: Another dry winter (230mm rain Oct-Mar, approx 60% average) lead to real worries about a major drought. Groundwater levels were low and Thames flows were similar to 1976. But then it rained: 2
2013: A cold spring: including 13th coldest March.
2013-14: St Jude Storm – 28 October. East coast surge – 5 December 2013. A mild winter following 10th warmest October. Started raining 13 December: 1½ feet rain in 2½ months = wettest 2½ months on record ... included the wettest Jan and 3rd wettest Feb on record.
2014: Broke the record as the hottest year in 356 years.
Dec 2015: With temperatures more appropriate for an average April, this was an exceptionally warm December. It was the hottest on record by a large margin. By January, Spring flowers (including daffodils and crocuses) were starting to emerge.
The last 20 years have been the hottest on record in central England.
There have been a lot of associated extremes - not all of them temperature related.
This graph shows how the 30-year long term average temperatures are projected to increase, as a consequence of climate change. The range (in blue) is for a low emissions future, whereas red shows the projections for a high emissions future by the 2050s.
Even the low emissions future is a massive change, and to a climate that we have not experienced before in this country. The extreme events associated with this will need some serious preparation to make sure we are ready to face them.
If we need to adapt to the extreme events associated with a higher emissions future, this will be much harder to do, more dangerous and more expensive.