How hard did it rain last week?

Hardcore Protonistas – and that’s you if you are reading this – will know that I love measuring things.

I love the way that measurements allow an extra level of wonder at the intricate detail of the world around us.

The rate of rainfall at NPL Teddington on 14th August 2014. The rainfall rate initially exceed 100 mm per hour. The first event resulted in 29.3 mm of rain and the second 18.4 mm of rain.

Last Thursday 14 August 2014 there were two torrential rain events at NPL.

They were kind of event that makes you stop what you are doing, go to the window and just stare. Personally they evoked memories of my childhood home and the security of being indoors and protected.

All very nice. But how much rain fell? And was it exceptional? These questions can only be answered by looking at data.

So I downloaded data from a weather station on the roof of one of NPL’s buildings. The graph at the top of the page shows the key features of the data.

• In both events the rate of rainfall was initially over 100 millimetres per hour
• The first event deposited 29.3 mm of rain and the second 18.4 mm of rain.

These data show that these were indeed powerful weather events.

Over the main NPL site, which comprises approximately 400 m  x 100 m = 40,000 square metres, the first event result in the deposition of approximately 1160 tonnes of water in approximately 50 minutes. Wow!

But were the events exceptional? The Met Office keep a record of extreme weather events (Link) which states that the most extreme UK rainfall events have been:

• Highest 5-minute total: 32 mm on 10 August 1893 Preston (Lancashire)
• Highest 30-minute total: 80 mm on 26 June 1953 Eskdalemuir (Dumfries & Galloway)
• Highest 60-minute total: 92 mm on 12 July 1901 Maidenhead (Berkshire)

Assuming these historic measurements are indeed reliable – which is not always the case – then the events in Teddington last week  were not technically ‘extreme’.

However they were astounding – and in the very best sense of the word – wonder-ful.

If Global Warming is happening, when will I be able to notice?

Probability of June, July and August average temperature anomalies in Moscow, Russia since 1950. This image shows that the average temperature in Moscow for summer 2010 was significantly hotter than in any year since 1950. Credit: Claudia Tebaldi and Remik Ziemlinksi. From Climate Central

What Is The Climate? A Science Song by Tom Glazer

Climate Change is slow on the scale of a human lifetime, and small in comparison with day-to-day and season-to-season variability. A change of average summer temperatures by 1 °C over 10 years is small compared to a day-to-day variability of typically ±5 °C in the course of a typical summer.

So we rely on meteorologist, and climatologists and statisticians to tell us when ‘something is happening’. However many people are understandably sceptical of the opinion of ‘experts’. In many areas – for example in the field of human nutrition – expert opinion is fickle and changeable. So at what point can we expect to be able to make up our own minds using our own senses?

That is the question addressed by James Hansen in a paper (Perception of climate change: pdf: open access) published by the National Academy of Science (of the United States of America).The paper considers extremes of weather and in particular, hot summer events in the Northern Hemisphere. I am not sure that the paper completely answers the question it sets out to answer, but nonetheless is it is very readable.

The conclusion of the paper is shocking. Compared with a reference period from 1950 to 1980, extremely hot summers in the Northern Hemisphere land area are now more than ten times as common. In a typical recent summer, these anomalous conditions now exist over 10% of the Northern Hemisphere land area, compared with a few tenths of a percent in the reference period.

Hansen concludes that these events (e.g. the current US drought , the Russian heatwave in 2010) are now so commonplace that they can they can definitely be said to be due to a changed climate. The extreme summer events arise not just from the warming trend, but also an increase in climate variability – essential extreme weather – compared to the locally-expected average weather. NASA have prepared a nice animation here.

Hansen is clearly convinced, and he convinces me. But other aspects of the changed climate conspire against the perception of change. For example if cold dry winters are replaced by not-so-cold but snowy winters, then people perceive those as being ‘worse’, even if the actual temperatures are higher.

Hansen is addressing this issue for a reason. It is not until the majority of people in a democracy become personally convinced that the Climate really is changing, that it will become inevitable that politicians will do something. Reducing greenhouse emissions – mainly carbon dioxide – is not fun for anyone. But if the US loses a significant fraction of its corn crop for more than a year or two in a row, then even the denialists and contrarians may think again.

Adapting to Climate and Climate Change

Picture of Brisbane in 1974

When we see weather extremes such as the current Brisbane floods, it’s easy to associate such a dramatic event with Climate Change – after all sea surface temperatures off the east of Australia have been in excess of 26 °C – and that’s where all the water has come from. However, as the above picture from 1974 shows, floods are nothing new in Brisbane, and this level of flooding appears to be roughly a 1 in 50 year event. In other words this is part of the normal climate of the area -or at least there is no obvious evidence otherwise.

And so the question that occurs to me is this: Why did people build a city in a place where they knew there would be floods? Now I know that a dam was built to reduce the risk of flooding, and so presumably things would be worse now if the dam had not been built. But nonetheless the question remains – is it smart to build houses in a place that catastrophically floods once in 50 years? I think the answer is ‘No’.

Now I suspect almost the entire population of Brisbane would disagree with me. In the same way, the people of New Orleans decided after Katrina that the answer to the question ‘Is it smart to build a city 18 metres below sea level in a Hurricane belt?’ was ‘Yes’. Why do people make such decisions? Well the answer is because is the individuals involved have lost everything they owned – and if they moved on and abandoned their old homes, then they would become effectively destitute. So staying makes sense for each individual. But eventually, in places where the flooding risk is so severe, and the costs of avoidance – in terms of dams and levees – so great, we will just have to say collectively: let’s move on. Because it seems pretty likely that Brisbane will flood again, and so will New Orleans.

So the problems of living in a marginally sustainable place such as Brisbane and New Orleans are severe. And there the question we are coping with is one connected with the frequency of extreme events in a particular climate. Now suppose that people decide to stay and spend 10 billion pounds making the place safe for, say, 100 years. How many times and how frequently are people prepared to spend that amount of money before it become cheaper to move on – to move higher. If climate change is active, many coastal communities may find that 1 in 100 year events happen once every 30 years – and then the economic and personal choice becomes more evenly balanced. And it will be many tens of years before we can definitely conclude that the climate has changed. But that is the nature of the challenge we face.

Now these are difficult questions to answer for rich countries such as Australia or the USA. For a poor country, such as Bangladesh, there is just nowhere else for people to go and then the prospect of increased frequency of flooding isn’t just an economic question, but a matter of life and death.