Professor Peter Stott:

I thank the committee for the invitation to attend. It is a great honour to be asked to present here. It was also an honour to be asked to present at the Citizens' Assembly, which I did last year, and I talked about climate change and its impact. My remarks today will follow on from the remarks that I made at the meeting of the Citizens' Assembly and the great discussions that I had there with those citizens about climate change and the challenges of climate change.

I will quickly say a few words about myself. I have worked on the Intergovernmental Panel on Climate Change. I was a co-ordinating lead author in the fifth assessment report and for the working group 1 part of that report, which is about the physical basis of climate change. My research interest is in looking at the causes of climate change. I also work at the Met Office Hadley Centre where we look both at the observations and the predictions of future change.

The first slide of my presentation asks the following question. What is the link between extreme weather events and either human induced climate change or natural climate variability? That was a question that the Citizens' Assembly, in particular, asked me to consider. It is one where, in scientific research, we have made quite a lot of advances.

This is important because citizens around the world are asking questions when they are affected by heatwaves, droughts and other extreme weather events of that sort. It is understandable and right that they are asking whether such events are a sign of climate change. People are asking scientists whether it is right to make a link between such events and climate change. Like everyone else, scientists are aware that there are natural variations in our climate. There have always been heatwaves. There have been floods and droughts in the past. It can be seen that the predictions which have been made over many years - going right back to the early reports of the Intergovernmental Panel on Climate Change from 1990, which made it clear it was expected that extreme weather events, such as heatwaves, floods and droughts, would happen with increasing frequency - have been coming true and playing out over the intervening years. The predictions that were made by the scientific community more than 20 years ago are becoming a reality before our eyes. Our observations are bearing out what the climate models have been saying.

I have provided some examples of this in the presentation I have furnished to the joint committee. The first slide following the headline slide refers to the question of a link between extreme events and human-induced climate change. It mentions the flooding in Ireland in the winter of 2015-16 and some of the storms that have been seen in Ireland. It includes a photograph of the railway line to Cornwall, in the west of England, which was washed away in January 2014. This happened close to my home in Devon. This slide also includes a map of the temperatures we saw in the summer of 2018, which was a particularly hot summer in Ireland and in the UK. The temperature anomalies depicted in the map show the difference between this summer's temperatures and average summer temperatures. The red parts of the map show the areas where temperatures were warmer than the average. It can be seen that this summer, temperatures in large parts of the northern hemisphere were much higher than average. There was a heatwave in Japan. There were extreme temperatures in northern Scandinavia, North America and Europe. The unusual thing about the summer of 2018 was that there were heatwaves in many parts of the northern hemisphere. In the UK, comparisons were made with and the heatwave of 1976, when there was an extended period of heatwave and drought. The UK Met Office made the point that this summer, weather conditions across the northern hemisphere as a whole were different from 1976. It was not the case in 1976 that there were warm temperatures throughout the northern hemisphere.

The third slide, at the bottom of the first page of my presentation, contains a graph based on a remarkable set of scientific observations and measurements that have been taken from the summit of the Hawaiian volcano Mauna Loa since the 1950s. The graph shows that carbon dioxide concentrations at that location are more than 400 parts per million, which is the highest level they have reached for at least 800,000 years, and have increased steadily by more than 2 parts per million by volume every year. There is a seasonal cycle in carbon dioxide concentrations due to processes in the land. In the spring, the biosphere takes up carbon dioxide. It is almost like it breathes it in during the spring and breathes it out during the autumn and winter. The reason for the annual cycle across the globe as a whole is that there is more land in the northern hemisphere than there is in the southern hemisphere. This means that the seasonal cycle for the north wins out relative to the south. It is a scientific fact that carbon dioxide concentrations are increasing. It has been well established for a long time that a natural greenhouse effect in the atmosphere keeps temperatures on the surface of the earth approximately 33° Celsius warmer than they would be if we did not have an atmosphere. Solar radiation from the sun acts to warm up the atmosphere more than it would if we did not have this natural greenhouse effect. Carbon dioxide is a greenhouse gas and through human activities, we are adding to this greenhouse gas and thereby enhancing the natural greenhouse effect and warming up the planet.

The next slide, which depicts average global temperatures, is based on a set of data compiled by the UK Met Office's Hadley Centre in collaboration with the climatic research unit at the University of East Anglia. It shows the average temperatures at the surface of our planet going back to 1850. We can construct reliable, instrument-based measures of global average temperature over this period by looking at readings taken from land-based thermometers and weather stations, as well as measurements of the temperature of the surface of the sea taken from ships. It can be seen from this slide that temperatures have been warming up. Temperatures are now approximately 1° Celsius warmer than the pre-industrial average, which is considered to be the average between 1850 and 1900. It is worth remembering that number when they think about climate change in the context of remarks I will make in a minute. It can be noticed from this slide that temperatures vary from year to year. The grey band in the graph represents our scientific uncertainty about making this measurement. While there is some scientific uncertainty, the 1° Celsius increase to which I have referred is way outside that uncertainty. As the intergovernmental panel put it, warming is unequivocal. It is of the nature of a scientific fact.

The next graph I have prepared is taken from the intergovernmental panel's fifth assessment report, which was published in 2013. It is another way of illustrating the average surface temperatures across the globe. There are fewer points marked on this graph because it depicts the average for every decade. The end of the graph shows that temperatures are approximately 1° Celsius warmer than they were at the start of the previous century. The variations that are seen when the figures are averaged across an entire decade are smaller than the year-to-year variations seen in the previous graph. We have averaged out some of the variations.

The blue band on the next graph relates to climate models that have been run in the past period in an attempt to see whether we can simulate what has been happening in observations if we include only natural causes of climate change when we run our climate models. We know that the solar output varies on an 11-year cycle. That has been included in these climate model simulations. Explosive volcanic eruptions occasionally eject dust way up into the higher atmosphere, which can temporarily cool temperatures. We also know that there are natural climate variations. Members may have heard about the El Niño southern oscillation, which occasionally means temperatures are warmer. All of those things have been included in the climate models represented in the graph to which I refer. It is clear from the graph that such factors do not explain the observed warming, which is well outside of anything that could possibly be accounted for by taking account of natural factors only.

The red band on the next graph shows what happens when the increase in greenhouse gas concentrations I have mentioned - it is now at 400 parts per million by volume in the atmosphere - is included in the climate models shown in the previous graph. Other human-induced factors, including factors associated with air pollution, aerosols and the effects of pollutants in the atmosphere, are also included for the purposes of this graph. It can be seen in this graph that the observed warming is well accounted for by these climate models when these factors are included in them. I referred earlier to projections that were made in the early 1990s on the basis of an earlier generation of climate models, which were not as sophisticated as those being used by the scientific community now.

What has happened since is consistent with what the scientific community was saying even back in the 1990s.

The next slide shows a couple of photographs from the plenary meeting I attended with governments to agree the fifth assessment report. The governments agreed with our scientific statement that human influence on the climate system is clear. I apologise as the next slide works a little better as an animation but I can describe what I seek to explain. It illustrates what happens with a warming world for extremes, such as heatwaves. We can think of pre-industrial climate, when climate was relatively stable, as it was for many thousands of years before the industrial era. We can plot temperatures in a particular place and see that some years were warmer and some were colder. Many years are average and only occasionally do we see particularly hot or cold years. We can then think about what happens if the climate is warmed. Even if climate is warmed by only 1° Celsius in the global average, which is where we are now, the probability increases significantly of having extreme temperatures in Ireland or Europe over a summer season. That is a property of the basic mathematics of the issue. What were very rare events have now become much more frequent.

I was involved with a pioneering study that looked at the European heatwave of 2003. It was fair to say this was something of a wake-up call for citizens of Europe as the European average temperatures were the highest they have been in the entire instrumental record of temperatures for Europe. This led to many tens of thousands of people dying in the heatwave, particularly vulnerable elderly people in urban centres. It was a wake-up call as maybe people did not realise how vulnerable elderly people could be in such a heatwave. We did a study, published in Naturein 2004, and found that as a result of this effect of warming on the risk of heatwaves, human influence had very likely at least doubled the risk of European summer temperatures being as hot as in 2003. We have come back to the work since and published a paper in 2015. We found the conclusion was robust. We have improved climate models and the study was still correct. We also found the risk of such heatwaves has continued to increase with further warming. Events that would occur twice per century in the early 2000s are now expected to occur twice per decade.

What about heavy rainfall and flooding? Storm Desmond had a major impact in December 2015. I have a graph that I showed to the Citizens' Assembly. It is a map produced in a collaboration between Met Éireann and the Met Office demonstrating rainfall anomalies, or how unusual were the rainfall totals in that month relative to a climatological average, which is often taken to be 1961 to 1990. The rainfall averages were extremely high in particular parts, including Ireland, north-west England and western Scotland and Wales. These were very extreme rainfall totals.

How can this be related to climate change? It is important to underline there was much variability in climate, and looking at rainfall in particular there is much variation. In our experience we know some months are particularly wet or dry, and there is much variation. There is an important scientific fact to be mentioned, which is that in a warmer atmosphere, there is more moisture. This is a rather basic physical property of how the atmosphere behaves as sea surface temperatures and the oceans warm, leading to more evaporation and the water holding capacity of the atmosphere increasing. That means there is approximately 6% to 7% more moisture in the air for every 1° Celsius of warming. I indicated earlier that it is approximately 1° Celsius warmer in the global average than in pre-industrial times, and that means there is approximately 6% or 7% more moisture in the air. That is what one would expect from physical principles, and it is what has been observed from satellite data. We have the observations to bear that out and it is what is happening.

What does this mean for extreme weather events and the risk of floods and droughts? In a scientific sense we must be a little careful as we must recognise there is much weather variation. The weather in this part of the world is very much associated with what is happening in the Atlantic Ocean and the storm tracks that bring storms across the Atlantic and hit us in our part of the world. We need to do some careful analysis and I thank Dr. Conor Murphy from Maynooth University for the result included in the documentation. He and his colleagues did an analysis of Storm Desmond in which they found evidence for this human fingerprint on this event. It is interesting as Storm Desmond was associated with what is known as an atmospheric river, a natural phenomenon that brings moisture from subtropical latitudes and transports it to the mid-latitudes, such as here in Ireland. We have seen such events in Cumbria, and parts of the world like New Zealand have also experienced the effects of atmospheric rivers. The point is that the analysis of Storm Desmond indicates that with warmer ocean temperatures and their effect whereby the atmosphere contains more moisture, there is a very much increased risk that when atmospheric rivers occur they will bring higher moisture than we had in pre-industrial times. Therefore, the impact of these atmospheric rivers is greater.

I will move to what this means for the future. The next slide has two graphs taken again from the fifth assessment report of the Intergovernmental Panel on Climate Change. In 2013, we agreed a conclusion with governments in the plenary. The statement indicates that continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system, while limiting climate change will require substantial and sustained reductions of greenhouse gas emissions. The graphs illustrate the point. The first shows global average temperatures, indicating how they are projected to increase with continued emissions and what might be referred to as a business as usual scenario. They show temperatures increasing by the order of 4° Celsius or more relative to pre-industrial levels. A change of 4° Celsius or 5° Celsius is significant. To put it in context, in the last ice age, when the climate was radically different from now and we had large ice caps covering many of today's inhabited areas, global average temperatures were only 5° Celsius colder than now. It a major shift in the climate. If emissions are reduced, warming will be very much less. The graph on the slide indicates the dramatic changes to Arctic sea ice that would result under continued emission rises. Under a business as usual scenario, there is the potential that there would be no Arctic sea ice at all in the summer.

I will move to what this means for Ireland.

I thank my colleague from the Irish scientific community, Dr. Conor Murphy from Maynooth, for this. We see a picture of drier summers, wetter winters and increasing temperatures in the summer. What is being done here is to relate the probability of the types of season that we had in the past. If one thinks back to the particularly dry summer here of 1995, one in every eight years could be as dry as that summer by the end of this century with continued emissions. Likewise, one in eight years could be as wet as the winter of 1994. The temperature of the 1995 summer could be once every seven years because warming would make that summer a relatively cool summer in the new climate. Therefore, only one in seven years could be as cool as that summer. It shows that with continued emissions what would once have been thought of as rather hot summers could become rather cool.

On the impact of climate change, this last set of graphs is taken from some work we did at the Met Office, and it looks at the impacts that could be avoided with mitigation. The basic point is that with continued emissions the risks of exposure to heatwaves and the risks to food security, flooding, water stress and water availability across the globe are significant and substantial. These graphs show that with continued emissions, and with continued business as usual, large factions of the population will be affected by substantial and significant impacts that would be difficult, if at all possible, to cope with. If the Paris Agreement of 2015, which states that governments should strive to keep warming to below 2° Celsius relative to pre-industrial levels and make efforts to keep warming to below 1.5° Celsius relative to pre-industrial levels, is successful, at least some of those impacts could be avoided. As I stated at the beginning, we are now already at 1° Celsius relative to pre-industrial levels. The Paris Agreement to limit warming to well below 2° Celsius needs to be thought of in that context.

To sum up, I restate the point that climate is warming, and that with increasing warming comes an increasing risk of some of the impacts I have talked about, namely, heatwaves, heavy rainfall, flooding and other impacts of climate change. Taking action to limit climate change will significantly reduce the severity of impacts on people and society.