Julia Slingo and Ken Cockshull summarize the data on climate change up to 2013 and discuss what effects it might have on UK gardens.

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Gardening climate science in a changing climate Julia Slingo and Ken Cockshull summarize the data on climate change up to 2013 and discuss what effects it might have on UK gardens.

1 Gardening climate science in a changing climate Julia Slingo and Ken Cockshull summarize the data on climate change up to 2013 and discuss what effects it might have on UK gardens. Our intention in this article is to present climate science in a manner relevant to gardeners. We will examine how climate change is affecting our weather. However, we recognize that it is quite difficult to do this in relation to the UK not only is our climate variable, but it also varies over the whole country. The south-west of the UK is known for its relative mildness whereas the Highlands of Scotland are much colder. Gardeners in the west are often blessed with an abundance of rain while those on the east are more likely to suffer from drought. So, how will the threat of global climate change affect us gardeners? This article is based on the RHS John MacLeod Annual Lecture given by Julia Slingo in November 2012 The global evidence for climate change It is perhaps necessary to briefly outline the evidence for climate change. The records for carbon dioxide (CO 2 ) levels in the atmosphere show that it is now more than 30% higher than at any time over the last 800,000 years. The evidence is that atmospheric CO 2 concentration remained at about 280 parts per million by volume (ppmv) for thousands of years until 1900 or so, and since then has steadily increased and reached March 2014

The Plantsman ppmv in 2013. The concentration of methane in the atmosphere has also increased in recent years and is produced largely by cattle, but also by the cultivation of rice.

2 The Plantsman ppmv in The concentration of methane in the atmosphere has also increased in recent years and is produced largely by cattle, but also by the cultivation of rice. Both CO 2 and methane are known as greenhouse gases. This means that when thermal energy from the warm surface of the earth is radiated back out towards space, these greenhouse gases are able to stop some of it from escaping. They act as a sort of blanket, keeping the earth warmer than it would otherwise be. So, where does the increase in these greenhouse gases come from? Records show that CO 2 emissions from land-use change were about 1.5 billion metric tons per year in 1960 and from fossil fuels were 2.5 billion metric tons per year. However, by the year 2010, CO 2 emissions from burning fossil fuels had risen dramatically to 9.1 billion metric tons per year while those from land use change had fallen to 0.9 billion metric tons per year and so now represented just 10% of the total. When we look at where all this CO 2 goes, it appears that about half of it is absorbed either by the oceans or by the biological activity of the earth, while the other half is left to accumulate in the atmosphere and so raise the CO 2 concentration. In essence, we are unlocking the ancient carbon that was laid down in the form of natural gas, coal and oil over many millions of years and we are releasing it into the atmosphere at an unprecedented rate. So far the rate of increase in global emissions of CO 2 continues to follow the high emission scenario (often termed business-as-usual ) used within the Intergovernmental Panel on Climate Change (IPCC) reports. The latest Fifth Assessment Report provides a stark reminder that to stay within the target of 2 C for global mean temperature rise we will need to keep our cumulative emissions of carbon since 1850 to within a trillion tonnes. It is sobering to realize that we have already emitted more than half that amount already. World climate change The impact of man-made increases in the concentration of CO 2 in the atmosphere is to raise the temperature of the surface of the earth. Figure 1 is a graph of the average annual global temperature for the warmest 50 years since 1850, ranked with the hottest to the left and the coolest to the right. To make it easier to interpret, we have identified each decade (or longer) with a separate colour. It is evident that the warmest years in the last 150 years are all clustered within the last two decades, and that all but one of the 10 warmest years has occurred since stands out as an exceptionally warm year and we know that this is due to the effects of El Niño, which tend to elevate global mean temperatures by at least 0.1 C. El Niño describes an Figure 1 Ranking of the annual global mean surface temperature in the HadCRUT4 dataset for each year since Years are colour-coded by time period, and the length of each bar indicates the observational uncertainty. (Source: HadCRUT4, Met Ofiice) Temperature difference ( C) from average Rank of hottest years to coldest March

climate science 5 4 3 Anomaly in C from 1981 1990 average 2 1 0-1 -2-3 -4-5 -6-7 -8 1911 1916 1921 1926 1931 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 2006 2011 2016

Its counterpart, La Niña, describes colder than normal conditions which in turn leads to a cooling of the global mean temperatures.

3 climate science Anomaly in C from average Figure 2 Mean UK temperature. Data are provisional from August (Source: Met Office) intermittent warming of the equatorial East Pacific Ocean, which has profound effects all round the world. Its counterpart, La Niña, describes colder than normal conditions which in turn leads to a cooling of the global mean temperatures. The effects of both are clear from the contrast between 1998 and El Niño events typically occur every three to seven years, although in the past decade we have experienced predominantly La Niña conditions. Interestingly, we can see one year from the 19th century, 1878, suddenly appearing among the records for 100 years later. This was also linked to El Niño. This was a devastating event for India where widespread famine due to failure of the monsoon, a consequence of El Niño, had serious impacts on what was then the British Empire. It is not surprising, therefore, that the IPCC 5th Assessment Report published in 2013 concluded that warming of the climate system is unequivocal and that greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years (predicted with greater than a 95% certainty). UK climate change A simple assessment is to look at how temperatures have changed over the last 100 years. One approach is to take the average temperature per month over a longish period, 1981 to 2010 in this case, and then look at how individual Anomaly (days) Figure 3 England and Wales growing-season-length anomaly, relative to average which is 295 days (c.9.5 months). (Source: Met Office) 52 March 2014

4 The Plantsman months in the years from 1911 to the present day differ (termed anomaly on vertical axis of Figure 2) from these long-term averages. In Figure 2, monthly averages that are lower than the long-term monthly average (represented by the continuous line) are in blue, while monthly averages that are warmer than the long-term average are above the line and are in red. Figure 2 shows two important things. Firstly, our climate is highly variable, and monthly temperatures can vary naturally from year to year by several degrees. Secondly, despite this large variability we can see a gradual warming in the last 30 years. This, of course, does not eliminate the possibility of cold winters, as occurred in 2010, but it does mean that they are less likely. A consequence of the warming that we see in the last 50 years is that, as most gardeners will agree from their practical experience, the UK growing season has got longer. We now frequently hear stories about plants coming into leaf or being in flower in early spring, or winter even, well ahead of the normal time for these events. The meteorological data confirm this trend. The meteorological definition of growing season is the period that begins when the daily mean temperature of more than 5 C occurs on more than five consecutive days and it ends when the daily mean temperature is less than 5 C for more than five consecutive days (after 1 July). From these data it appears that the average growing season in the UK is now about one month longer than it was in Figure 3 shows this with the more prevalent positive anomaly in recent years. As most gardeners already know, the increase in the length of the growing season is not uniform over the entire UK and we can all be surprised by a late frost. Indeed, the main risk associated with precocious leaf emergence and flowering is the occurrence of a late spring frost. UK meteorological records show that average temperature has risen in early spring and we can conclude that air frosts in spring are less common. There are now typically four fewer frost days in spring across England and Wales than in But they have not gone away and when they occur, they can cause havoc for gardeners. That is because, although our climate is warming very gradually, natural variability is still very large and that will continue to dominate our climate for many years to come. For example, in 2013 the spring was exceptionally cold and we have to go back 50 years to find a spring that was as cold (Figure 4). When we look at rainfall, however, it is not obvious that there has been any consistent trend in monthly rainfall since 1911; each month is as likely to be higher than the monthly long-term average as it is to be lower (Figure 5). So, monthly rainfall is just as variable as it has always been. But, because the average temperature between May and September is increasing, water will evaporate from the ground more rapidly and the reservoir of water in the soil will be depleted more quickly. One consequence is that the plants will run out of water more Highest average Smoothed kernel filter Most recent value Mean temperature ( C) Lowest Figure 4 Mean UK temperature, March. All values are final. (Source: Met Office) March

climate science 300 275 250 225 Percentage of 1981 2010 average 200 175 150 125 100 75 50 25 0 1911 1916 1921 1926 1931 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 2006 2011

This combination of factors makes one wonder whether the conventional lawn will have a place in the gardens of the future. However, the graph of monthly mean rainfall hides some important messages.

5 climate science Percentage of average Figure 5 UK rainfall anomaly. Data are provisional from August (Source: Met Office) rapidly in summer and suffer more readily. This combination of factors makes one wonder whether the conventional lawn will have a place in the gardens of the future. However, the graph of monthly mean rainfall hides some important messages. When the data are split into winter months (October to April) (Figure 6) when ground water is typically recharged, and summer months (May to September) (Figure 7) when ground water is discharged, long-term (20 years or more) variations that have important consequences for gardens are evident. From the 1980s onwards the UK has had wetter winters and drier summers than in the 1950s and 1960s. Some of these changes appear to be due to periodic oscillations in the temperature of the North Atlantic Ocean, known as the Atlantic Multi-Decadal Oscillation. It is possible that, since 2000, we are moving back into a period of drier winters and wetter summers. Another potentially important aspect of global warming is that heavy rain events will become even heavier. That is simply basic physics which tells us that warmer air can hold more moisture. A trend towards much heavier rain events has already been detected in some parts of the world, including India and China % Figure 6 UK rainfall October to April, % of average. (Source: Met Office) 54 March 2014

The Plantsman % 160 150 140 130 120 110 100 90 80 70 60 50 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Figure 7 UK rainfall May to September, % of 1961 1990 average.

Simon Garbutt Predicted milder temperatures in spring will lead to earlier flowering, but blossom such as this pear will still be at risk from frosts during cold spells in late spring But what about

6 The Plantsman % Figure 7 UK rainfall May to September, % of average. (Source: Met Office) alternative way of looking at this is to say that the amount of rain occurring in the 1 in 100 day event is increasing. Simon Garbutt Predicted milder temperatures in spring will lead to earlier flowering, but blossom such as this pear will still be at risk from frosts during cold spells in late spring But what about the UK? A new analysis (Figure 8) of daily rainfall shows that the likelihood of heavy rain is probably increasing, especially in summer. Figure 8 shows that March 2014 heavy rain events that, on a long-term basis occur once every 100 days (often referred to as the 99 th percentile), are now occurring more frequently, closer to once every 75 days. An The future Where is all this leading us? Global temperatures have risen by 0.8 C since They are predicted to continue to rise, with the potential to be warmer than today by as much as 4 C by the end of the century. Fortunately, for the UK we will be buffered from the worst of that because of where we sit, next to the moderating effects of the North Atlantic Ocean. Nevertheless, the UK Climate Projections published in 2009 (UKCP09) predict that there is a high probability that 30-year mean summer rainfall will decrease by the 2080s, potentially by as much as 20 50%, with only a very small probability that it will increase. Similarly, there is a high probability that mean summer temperatures will rise by as much as 4 C in the 2080s. However, new research looking at natural variability into the future 55

7 climate science has shown that our climate will be just as variable, if not more so. For example, really wet summers are just as likely, but very dry summers will become more frequent. The effect on gardeners The main take-home message for gardeners in the UK is that we have learned to cope with highly volatile weather and climate. Indeed, gardeners, above all, recognize that this has shaped our gardens in the past and that it will continue to shape them in the future. Nevertheless, we do need to recognize that the effects of global warming are beginning to be seen in longer growing seasons, fewer frosts, and, probably, in more extreme rainfall events. This means that we are likely to experience milder temperatures in February and March. This will help extend the growing season by up to a month. The milder temperatures of early spring will stimulate plants into leaf earlier and even to flower earlier. However, this will then expose them to the chance of frosts during cold spells in mid and late spring. These late frosts will not be eliminated by global warming. The trend towards milder temperatures also means there will be fewer days of frost that will kill off pests and diseases. Therefore, infestations and diseases may survive winter and begin earlier in the growing season and become more prevalent. Indeed, new pests and diseases may spread into the UK from warmer climates as our climate becomes more suitable for them to survive. Although the amount of rain we get in a year seems unlikely to change dramatically, it looks more likely that there will be more days with very heavy rain and so correspond ingly longer periods of drought in-between. It is these changes that make the future of our traditional lawns look doubtful, at least in the absence of irrigation. The greater likelihood of extreme rainfall in summer also poses dangers for plants and for the structural elements of gardens. How we gardeners should respond to climate change is complex, but the evidence is that we do not need to rush out and plant cacti! It is worth remembering that, even as our climate warms and the growing season lengthens, we will not see an increase in sunshine, which is needed just as much for plant growth. So, there will still be limits on what we can plant. Also, we will continue to live in a highly variable climate; there is no doubt that variations such as the multi-decadal changes in the North Atlantic Ocean and other phenomena will continue to shape our gardens. Gardening in the UK has never been easy, and it is probably fair to say that it is now getting that little bit harder. Prof. Dame Julia Slingo dbe is Chief Scientist of the UK Meteorological Office in Exeter, Devon. Dr Ken Cockshull is a member of the RHS Science Committee and an Associate Fellow of the University of Warwick Crop Centre. Figure 8 Extreme daily rainfall statistics. (Source: Met Office) 56 March 2014

What is the IPCC? Intergovernmental Panel on Climate Change

IPCC WG1 FAQ What is the IPCC? Intergovernmental Panel on Climate Change The IPCC is a scientific intergovernmental body set up by the World Meteorological Organization (WMO) and by the United Nations