Professor Sonia Seneviratne:

I will keep my presentation short because I have a lot of material. I thank the committee for the invitation to attend. It is a great honour to be asked to present here.

I will talk about climate extremes at global warming of 1.5° Celsius and 2° Celsius. I shall provide a little more background on projected changes at those two levels of warming. I was involved as lead author on the chapter in the special report dealing with the impacts of global warming of 1.5° Celsius on natural and human systems.

I want to start with a point that Professor Valérie Masson-Delmotte made, namely, that we are living in a warmer world, with 1° Celsius of global warming. We saw very well this summer that the implications of warming of just this amount are substantial. There was a drought and heatwave in Ireland, and there was also a heatwave in the United Kingdom. There were forest fires in Sweden. At the same time, there was a drought in Germany and Switzerland, where I am from. There were fires in California, and more again recently. There were heatwaves in Japan and Canada. Any single one of those events would be difficult to attribute to climate change but it is unlikely that all the events would have happened at the same time without climate change.

I want to mention a point made that was quite striking for me. Often when we think of the impact of climate change, we think of developing countries but there were several deaths associated with heatwaves in developed countries, including in Japan and Canada, which are developed countries in high latitudes or mid-latitudes. Recently there were deaths in California. Even developed countries are not safe from climate change.

Members can view a slide illustrating the widespread heat wave. I believe Professor Stott mentioned this a month ago. In the past, there were areas that might have been as warm locally as this summer but what was very different this summer is that there was a global signal. It was not located only in Europe.

If we examine temperature anomalies this summer by comparison with the middle of the 20th century, we see that from April to July this summer, there was an anomaly of more than 2° Celsius. It means that even those there is warming of 1° Celsius, there can be much larger anomalies in a single location or in single months. One can see clearly that this was a very extreme summer.

It is not only temperature that is affected; so too is the water cycle. There are impacts in the form of heavy precipitation. The link is that when there is warmer air, it can contain more moisture, which means that when it is raining, there is more rain. There were some floods in France in 2016. There were some in Japan in 2018. There were some in the United States associated with tropical cyclones. I have heard about flooding in Donegal. There is no specific study on this but with heavy precipitation increasing with warmer climate, it is probable that precipitation levels have also been affected.

To give members a general picture, the last IPCC report, from 2013, shows a likely increase due to human influence on climate. For heavy precipitation, the evidence is less strong but there is medium confidence that the trend since 1950 is also due to human influence. There was lower confidence in the evidence for drought and tropical cyclones but there is new evidence for the attribution of drought to human-induced climate change, for instance, in the Mediterranean region. This is also the case with heavy precipitation associated with tropical cyclones. We have more evidence for extremes being linked to human-induced climate change.

I refer to extremes associated with warming levels of 1.5° Celsius and 2° Celsius. I wish to demonstrate the direct link between carbon dioxide emissions and global warming. Members can see from the slide that, for a given warming target, there is an associated CO2 emissions level. We are speaking only of global warming, not of regional changes in extremes. A slide shows the average change of temperature of the hottest days with average warming of 1.5° Celsius and the change on the coldest nights. On average, while there is average warming of 1.5° Celsius, it is much higher in many areas, especially land areas. Land areas tend to warm much more, to a factor of two or three. This is why, when one hears about warming of 1.5° Celsius, which sounds like a lot, one should realise, it can imply warming of 2°, 4° or 5° Celsius in some locations. Europe is strongly affected.

The same principle applies for warming of 2° Celsius but there are even greater changes on the hottest days and coldest nights. A slide shows the average warming is between 2° Celsius and 3° Celsius but it is up to 4° Celsius in some cases - for the hot extremes. In the high latitudes, there is warming of up to 6° Celsius or 8° Celsius.

We considered the difference between changes with warming of 1.5° Celsius and 2° Celsius in the report. We could identify robust and statistically significant differences. There are differences in the temperature of hottest days at the top and the number of hot days. In most locations, including Ireland, we find substantial differences in hot extremes based on warming of 1.5° Celsius and of 2° Celsius.

Another slide shows changes in mean precipitation. There is a trend towards a decrease in precipitation in the southern part of Europe, for instance, but an increase in the northern part of Europe. Ireland is located in an area where there is a substantial robust increase in mean precipitation with additional warming.

Mean heavy precipitation is projected to increase compared to pre-industrial levels, both at 1.5° Celsius and 2° Celsius. It is the same with extreme precipitation, with an increase in heavy precipitation at 1.5° Celsius compared to pre-industrial levels and at 2° Celsius compared to pre-industrial levels.

There are some differences at 1.5° Celsius and 2° Celsius. The slide shows results aggregated for all of northern Europe, which includes Ireland. On one side, we can see different indices of heavy precipitation, and, for most measures of heavy precipitation, there is statistically significant increase in heavy precipitation at 2° Celsius compared to 1.5° Celsius.

I refer to some other impacts associated with global warming. We look at impacts on human, managed and natural systems above 1.5° Celsius. In some cases, we have irreversible changes. For example, in regard to coral reefs, we are at a warming level that is risky for the reefs and could lead to extinction if there are higher levels of warming. There are also irreversible changes between 1.5° Celsius and 2° Celsius with melting of ice around Greenland and some sea level rises.

To conclude, half a degree matters. Limiting global warming to 1.5° Celsius would avoid widespread increases in extremes, as well as heavy precipitation extremes in several regions and droughts in some regions. We are experiencing the impacts of global warming at 1° Celsius. We have amplification of regional changes in temperature extremes and there has been a much higher warming of extremes compared to the mean global temperature in many locations, as well as, for example, some irreversible changes for biodiversity and sea level rise. There are also substantial changes in heavy precipitation and in temperature extremes for Ireland and northern Europe. Of course, we need to model scenarios to have more information for Ireland. As was mentioned, limiting global warming to 1.5° Celsius is geophysically still possible but it would require unprecedented changes.