Climate change, weather elements and natural disasters: what links?Sep 16, 2022
Over the past couple of years, the idea that humanly-created climate change is real and worrying has become increasingly accepted in Australia. Few scientists now argue the opposite point of view, the commentariat has largely followed suit even News Corp’s opposition is no longer a matter of policy and polls suggest that scepticism and denialism are in retreat in the popular mind.
Meanwhile there is much concern in 2022 about unusually severe droughts and historically low river levels in Europe, North America and China alongside severe flooding in Pakistan due to rapid glacier melting and unusually high monsoon rains. An important question is whether such experiences are simply ‘outlier’ extremes or responses to secular trends in weather phenomena related to changes in climate itself.
Nothing here is simple. Yet some things are clear, and climate scientists have for more than 20 years been largely united in the view that increases in greenhouse gas concentrations are amplifying the extremes of wet and dry in our climates. Hence, they suggest, natural disasters due to floods, bush (wild) fires and droughts are all being (or will be) exacerbated.
The central scientific explanation for such trends is the rise in carbon dioxide levels, which have increased from about 280 parts per million to nearly 420 since the early nineteenth century a very rapid proportionate rise of almost 50 per cent in only 200 years. Most of the increase has taken place since about 1960, suggesting that it is occurring at a rising rate. In response, atmospheric and ocean temperatures have risen and thus the Greenland and Antarctic ice sheets and the world’s glaciers have experienced strong melting and average sea level has risen. These are simple observed facts, well established empirically. They are not contentious.
The human role in contributing to these trends remains a matter of debate but doubt is declining. Massive increases in the spread of industrialisation since the middle of the nineteenth century, along with the growth and burgeoning material consumption of the global population and the large-scale removal of forests especially in the tropics, are well recognised as playing causative roles in relation to rising greenhouse gas levels. Other potential causes of temperature increases for example increases in solar output or changes in the earth’s orbit around the sun have been considered and rejected. Gradually, it seems, the pillars of climate change denialism are being dismantled, although elements of it persist. Some people simply cannot believe that mere humans can affect something as vast as the atmosphere.
The impacts of climate change on weather-related phenomena appear to be quite variable. There are great differences between phenomena: some show the climate change signal sharply while others do not. In Australia, for example, snowfall has clearly declined over the relatively long term of several decades. Evidence of this decline in the Snowy Mountains over the past 70 years is incontrovertible, notwithstanding its unevenness as a trend: snow seasons have shortened (particularly in the spring weeks), depths of snow accumulation have declined, the frequency of ‘bumper’ snow years has fallen and the area covered by snow in winter is less than it used to be. Skiable snow at low altitudes, as at Kiandra and at the site of the old Kosciusko Hotel below Perisher (as well as in the Barrington Tops on the northern edge of the Hunter Valley), has virtually ceased to exist as areas subject to snow lasting on the ground have diminished. Snowlines are clearly receding.
Anecdotal evidence going back to the early twentieth century also suggests that big Australian snow years were once more frequent than has latterly been the case. They may have occurred on average slightly more than once per decade between 1910 and 1980, but in the Australian Alps there has not been a genuinely big year since 1981. Our ski industry, seemingly, would be in a parlous state were it not for substantial investment in artificial snow-making over the past 30-plus years.
Evidence strongly suggests that some weather-based natural hazards are becoming more problematic as far as their implications for human beings are concerned. In particular the bush fire problem in Australia (and indeed globally) is clearly growing worse. Fire seasons are starting earlier and ending later, leading to an overlap in the fire seasons of the two hemispheres and creating challenges for the transfer of fire-fighting resources between them. The number of ‘dangerous’ fire weather days has increased, extreme fires are running hotter, the areas being burnt in the more severe seasons are increasing in size, and rainforests not known in the past to have burnt are becoming affected. These changes do not appear to have been confined to the extreme bushfire season of 2019-20 in south-eastern Australia: in other words they have not been confined to a single extraordinary fire season.
There is no doubt that the spread of settlement into areas that are prone to bush fires has contributed to increased consequences including economic costs and deaths. The Adelaide Hills, the Dandenong Ranges, the Blue Mountains and elsewhere are obvious cases as a result of both suburban development and the ‘tree-changer’ phenomenon but the trends towards greater frequency and intensity of fires clearly go beyond the impact of changed settlement patterns. Higher temperatures and the resultant greater rates of evaporation are apparently causally involved.
Heatwaves in Australia have increased in frequency, duration and intensity since about the 1950s, with numbers of deaths reflecting the trends. For droughts there are suggestions of increasing frequency, duration and intensity in many parts of the world. Increases in the frequency of very hot days in Australia may be a factor intensifying drought conditions.
For some hazards, the picture seems to suggest little long-term change in weather elements. Tropical cyclones, globally and in the Australian region, do not appear to be increasing in frequency or intensity. Much the same is probably true of convective thunderstorms that produce large hailstones or strong winds. Extreme events occur periodically in the case of such weather systems, as has always been the case, but temporal trends in their occurrence are difficult to discern.
Rainfall trends are variable, especially when viewed at a regional level. In south-western Western Australia, rainfall totals have decreased substantially over the past several decades, especially during the cooler months, as has been the case to a degree in Victoria. There are potential implications for urban water supplies. Probably the declines have resulted from frontal systems being pushed southward in a warming world. Meanwhile large parts of northern Australia have experienced increases in average precipitation as might be expected given gradually warming adjacent seas.
Now take floods. The evidence is clear in Australia and elsewhere that intensification of short-term (less than 24-hour) rainfalls has increased, leading to more flash flooding, but establishing trends for riverine flooding is difficult. It is much easier to demonstrate that we have made the flood problem worse by exposing more people to flooding settlement patterns, again than by making flooding more frequent or more extreme in terms of depths and volumes of floodwater in individual events.
Extreme floods have struck periodically, of course. In New South Wales Windsor (1867), Maitland (1955), Nyngan (1990) and Lismore (2022) are notable examples of floods reaching much higher levels and/or containing substantially greater volumes of water than have been recorded on other occasions in those locations. But there appears to be no incontrovertible tendency towards extreme floods occurring more often on our rivers than in times past.
Yet the scientific consensus appears to be that large floods are becoming rather more frequent (which is to say that Annual Exceedence Probabilities for the bigger events are increasing) while small floods are becoming less common. But this tendency is probably not strong, or at least not yet.
Overall we have tended to make rainfall more ‘efficient’ in causing flooding (especially in cities by virtue of the hard-topping of greater proportions of the ground), and we have made that flooding more consequential to humans. Removing vegetation from floodplains to create farms, and creating whole towns and suburbs on floodplains, have contributed greatly to increasing the impacts of floods on human interests. But it is harder to see the signal of climate change than the consequences of the substantial environmental changes we have instituted.
There is little doubt, though, that coastal and estuarine flooding are being and will increasingly be exacerbated by sea level rise. Likewise the potential for coastal erosion and shoreline retreat, a serious problem on Australia’s populated east coast, must increase with higher sea levels and further coastal development. Meanwhile the increased capacity for a warmer atmosphere to hold more water vapour and for the greater energy of that atmosphere in triggering intense rainfall drivers like east-coast low-pressure systems cannot do other than increase flood intensities at some stage if they haven’t already done so. Momentum in the underlying causes for intense rainfall has been established and is building further. This does not bode well for the times ahead.
The evidence of the impact of climate change on our flood problems is thus not entirely clear. But that impact is very likely to be one of intensification, at least prospectively if it has not been demonstrated already. While we must obviously seek deeper understanding of the complexities in these matters, we must surely be at the point of recognising that, on the balance of probabilities, humanly-induced climate change is to a degree contributing to the flood problems we confront. Moreover, it is hard to see that tendency not increasing in the future.
The worst of all this is that climate change may be creating extreme weather events more frequently, whereby they occur simultaneously or closely following one another (even in the same areas) with ‘compounding’ implications in terms of the problems that result. There are vital lessons here for public policy.
We must overcome ‘policy diffidence’ in these matters. Sadly, of course, such diffidence has been all too common in relation to both climate change and the management of weather-related disasters in Australia.