Climate change and its consequences: What do we know? What do we assume?
Following weather disasters like those that recently hit parts of the USA and several Caribbean countries, the first question many people ask is: Did climate change play a part in this?
Scientific opinion is almost unanimous that climate change is predominantly caused by human activity and that it influences weather extremes like windstorms, hail, torrential rain and heatwaves.
But the interrelationships involved are extremely complex: Natural climate oscillations exert a major – and so far much greater – influence on weather-related disasters, and the vulnerability of buildings and infrastructure to natural events also has a major effect on the scale of losses.
Some questions are still easy to answer. For example:
Are there completely new kinds of natural disaster that would not have been possible without climate change?
Can individual events be attributed to global warming?
Or does climate change even influence major earthquakes?
What else do we know or assume?
- It is virtually certain that global warming since the middle of the 20th century has been primarily caused by humans. Although fluctuations in the sun's radiation intensity were partly responsible for warming in the first half of the 20th century, they do not explain the intense warming since then. Over the last few years with the steepest increases in temperature, the sun's radiation intensity has actually declined.
- All 16 years since 2001 rank among the 17 warmest since measurements began. The last three years (2014–2016) were the warmest ever.
- Greenhouse gases, most notably carbon dioxide (CO2), absorb a portion of the Earth’s longwave radiation, which would otherwise be emitted into space. This increases the energy content of the lower atmosphere (troposphere), which therefore heats up. As well as the air, the ground and oceans also become warmer. Part of the energy emitted back towards the Earth’s surface is not emitted towards higher layers of the atmosphere, so the lower stratosphere cools. Both trends, the warming of the lower and the cooling of the upper atmosphere, have been measured for decades: they represent a fingerprint of the greenhouse effect.
- The “weather motor” is running faster: More water evaporates from warmer oceans, and a warmer atmosphere can absorb a greater volume of water vapour. Both these factors increase the potential for heavy rainfall. The increased amount of water vapour in the atmosphere means more energy, which is released when it condenses into clouds. It is this energy that fuels thunderstorms and tropical cyclones. But on a regional level, climate change is also likely to lead to a greater number of droughts.
- As documented in the latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the vast majority of scientists expect an increase in the number of weather-related natural disasters as a result of climate change. The trend will vary considerably according to region and type of event. Statistical evaluation is complicated because, thanks to better documentation, the web and social media, more events are now being recorded than ever before. In our NatCatSERVICE database, we provide statistics that exclude small weather events, but which nevertheless contain a degree of reporting bias. It is striking that, while there are no clear upward trends for geophysical events like earthquakes and volcanic eruptions, there certainly are for weather-related events.
Natural hazards and indications of climate-change influence
- Severe thunderstorms in North America: There has been a significant tendential increase in losses from severe thunderstorms with hail, wind gusts, tornadoes or torrential rain, even after the figures have been adjusted for rising values. This trend tallies with changes in meteorological data, and in particular the increase in water vapour content in the lower atmosphere. Since the additional water vapour comes from the warmer oceans, this suggests a connection to climate change. And this also correlates with what scientists expect as a consequence of climate change.
- Severe thunderstorms in Europe: As in North America, normalised losses have increased significantly here. Studies show that, in parts of Europe, including southwest Germany, severe thunderstorms with hail have already become more frequent, as has severe hailfall. Greater loss susceptibility of buildings fitted with sensitive thermal insulation, solar installations and expensive metal façades is also driving up loss potentials. The costliest single thunderstorm event since 1980 occurred in Germany in July 2013, when powerful hailstorms in particular caused overall losses of €4.6bn in today's values. About €3.5bn of this was insured.
- Heatwaves, heavy rainfall and the jet stream: Several studies see a connection between persistent weather patterns in the northern hemisphere and the excessive warming of the Arctic in summer that is being driven by climate change. This increases the likelihood of heatwaves or persistent heavy rainfall with flooding. Examples include the heatwaves in Europe in 2003 and 2010 (Russia), and the severe floods in Europe in 2002 and 2013.
- Monsoon and heatwaves in South Asia: In South Asia, and particularly in India, heatwaves are now the events that bring the most drastic humanitarian consequences. Thousands of people die in summer heatwaves, when temperatures can reach well over 40°C. Scientists assess it as “probable” that heatwaves in South Asia will become more frequent and more extreme as a result of climate change. The IPCC also expects climate change to influence monsoon rainfall. In India, average rainfall during the monsoon period is likely to increase slightly, as will fluctuations between seasonal precipitation highs and lows, which would pose a huge problem for agriculture. At the same time, extreme rainfall events are expected to increase. In summary: higher seasonal variability and more extremes.
- River flooding – Uncoupling of meteorology and losses: In Europe, the trend in losses after adjustment for increases in values has fallen. But scientists are generally convinced that climate change is influencing precipitation patterns in Europe and will produce heavy rainfall more frequently – in conjunction with a tendency for longer dry phases to occur. Over the last few years, there have been several examples of severe river flooding, for example in 2013 in the south and east of Germany and in neighbouring countries. In terms of its meteorological/hydrological characteristics, the flooding was similar to the Elbe floods of 2002, but caused a third less damage. This can no doubt be explained by the billions invested in flood protection in Germany and neighbouring countries following the Elbe floods of 2002. Money well spent!
- In North America and China too, there are indications that massive investment in flood protection on rivers has led to a reduction in normalised losses, even though there has been a greater number of destructive events.
- Hurricanes in the North Atlantic: Concrete predictions are difficult here. As a result of climate change, scientists expect no increase in the overall number, but a higher proportion of severe storms. The background: As a result of global warming, higher layers of the troposphere could become drier. Convective processes that are essential for the formation of tropical storms are inhibited. However, at the same time, the energy potential increases along with the higher water temperatures. Thus, any hurricanes that develop in future could tend to reach higher intensities.
But in the tropical North Atlantic, storm activity is heavily influenced by natural climate cycles, such as the persistent warm and cold phases in the North Atlantic over many decades (Atlantic Multidecadal Oscillation), or by teleconnection effects from the ENSO climate oscillation in the Pacific – better known as El Niño and La Niña.
Greater protection against the consequences of natural disasters is urgently needed to prevent the unchecked growth of their humanitarian and financial impact. Greater insurance cover can go some way to cushioning the financial losses, which would clearly benefit the individuals and economies affected.