By Matt Burdett, 31 March 2018
On this page, we look at the impact of climate change on the incidence and severity of extreme weather events, including drought, from the perspective of changes to environmental systems.
What is the difference between climate and weather?
Weather and climate are often confused. Climate is the “long term average of the weather”, while weather is “the state of the atmosphere at any given time and place” (EPA, n.d.a). Some examples of the difference are shown in the table below.
A temporary lack of rain that causes a drought in a place that normally has water.
A long term lack of rain that creates dry areas such as deserts.
An individual tropical storm (such as a hurricane).
The tropical storms that a location such as the southern USA typically experiences in a normal year.
A cold day in December in Frankfurt, Germany.
The cold weather that is normally experienced in the winter.
Therefore an ‘extreme’ weather event is any situation in which the weather is much more severe than usual. As climate is the average or typical weather in a place, it means that extreme weather events contribute to the overall climate. “Climate is defined not only by average temperature and precipitation but also by the type, frequency, duration, and intensity of weather events such as heat waves, cold spells, storms, floods, and droughts.” (EPA, n.d.) Extreme weather incidents are those that are outside the normal expectation for that type of event. For example, Hong Kong experiences three or four severe tropical storms (called ‘typhoons’ locally) in a typical year. But only a typhoon that was significantly stronger than normal would be considered ‘extreme’.
These extreme weather events can be analysed from two aspects:
Incidence relates to the number of these events that occur in a year. Severity refers to the size of the event, which can be measured by weather features (e.g. the speed of the wind in a tropical storm, or the amount of rain that falls in a day) or by the impacts (e.g. number of injuries, number of deaths, or number of homeless).
Extreme weather: The big picture
Overall, the incidence and severity of extreme weather events is increasing and predicted to rise further. The map belows shows some forecasts of the likelihood of extreme precipitation and heat. The study by Fischer and Nutti (2015) found that about 18% of the current extreme precipitation events and 75% of the current extreme heat events are due to global climate change since the beginning of the industrial period, and are therefore linked to human activities.
- Expected changes in extreme precipitation and heat at different levels of climate change. Source: Fischer and Knutti, 2015.
It is very difficult to determine whether a specific event was due to human-induced climate change. The study above attempts to determine this for global trends, but many other studies have also looked at events and tried to ascertain whether the event was caused by humans. This is known as “extreme event attribution”. The Carbon Brief website has looked at over 140 such studies and produced an interactive map showing the results (Pidcock and Pearce, 2017). The authors draw an important conclusion:
“Scientists need to examine the circumstances of each individual weather event. It’s only by combining evidence from all around the world that they can begin to draw broader conclusions. The evidence from the scientific literature – as it stands – suggests that of the extreme weather events scientists have studied so far, climate change has made more than 60% of them more likely or more damaging.”
Above all, they note that it is not possible to blame a specific event entirely on climate change, but it is possible to assess whether climate change was a contributing factor to the event.
- Attributing extreme weather to climate change. Source: Pidcock and Pearce, 2017.
Extreme weather events: incidence
The number of extreme weather events worldwide is increasing, but it is not the same for all types of extreme weather event. Extreme heat is becoming more common, but some other types of weather event show a more complex picture. Furthermore, some areas of the world are experiencing a change in extreme weather more than others.
Example of extreme weather: heat in Australia
A good example of the increase in extreme weather is that of heat. The Australian Bureau of Meteorology reports an increase, as shown on the graph below. “Extreme days are those above the 99th percentile of each month from the years 1910-2015. These extreme events typically occur over a large area, with generally more than 40 per cent of Australia experiencing temperatures in the warmest 10 per cent for that month. The frequency of very hot (greater than 40°C) daytime temperatures has been increasing since the 1990s.” (Bureau of Meteorology, 2018).
- Number of days each year where the Australian area-averaged daily mean temperature is extreme. Source: Bureau of Meteorology
However, it’s not happening everywhere at the same strength. The northern coast of Australia and the central-eastern areas have experienced a greater number of extremely hot days, as shown on the map below. For example, the area around Darwin (the darkest area to the far north) has experienced an increase of over 12 more days per year over 35°C (note the scale is slightly confusing and is explained in the caption). Some areas have even experienced a decline in the number of extremely hot days. This is because local conditions are not the same: things like vegetation coverage, wind speed, wind direction, available moisture, seasonal river flow and storm incidence can all affect how temperatures behave on a local scale.
- The trend in annual number of days per year above 35 °C from 1957–2015. An increase of 0.2 days/year since 1957 means, on average, that there are almost 12 more days per year over 35 °C. Source: Bureau of Meteorology, 2018.
Extreme weather events: severity
While some types of extreme weather are becoming more common, some types are occurring just as often as they used to, but are becoming more severe. As with incidence, it’s not possible to identify any single event as being due to climate change but it is possible to identify climate change as a contributing factor to the overall trend.
Example of extreme weather: Hurricanes in the North Atlantic
A good example of the complexity of extreme weather is to look at the number of tropical storms in the North Atlantic, because there are good records of this type of weather event going back almost 150 years. The graph below shows that the number of recorded hurricanes (the local name for a severe tropical storm that affects the USA) peaks in the early 2000s. However, it’s not a straight-forward picture. In the early years, some hurricanes weren’t recorded because they never made landfall. When these ‘missing’ hurricanes are included, it appears that the frequency of hurricanes has gone down or even that there is no particular overall change.
- Number of hurricanes in the North Atlantic. Source: EPA, n.d.b.
However, the number of hurricanes shows only the incidence. The severity of the storms has increased. Severity is measured by not just frequency but also duration and strength. These are combined into the Power Dissipation Index, which is shown on the graph below. It suggests a clear increase in the relative power of hurricanes over the period 1949 to 2015.
- Power Dissipation Index values for hurricanes in the North Atlantic. Source: EPA, n.d.b.
Major hurricanes such as Hurricane Katrina (2005), Hurricane Sandy (2012) and Hurricane Harvey (2017) have all grabbed headlines and made people ask whether this is due to climate change. The answer is that while each storm may have occurred without human induced climate change, the energy for a hurricane comes mainly from the ocean. Higher ocean temperatures mean more energy is available for the storm, and therefore a potentially higher intensity of hurricane.
Drought: incidence and severity
Drought can be defined in many ways, but for the purposes of this page, it is “an extended period of deficient rainfall relative to the…average for a region” (Graham, 2000). Climate change is expected to increase drought due to the rise in temperatures which allows air to absorb more moisture. It is not only about rainfall, however. The increased temperature will cause more evaporation, which will remove more water from the soil and make it harder for plants to grow.
Ironically, drought can lead to flooding. When the ground is very dry it becomes very hard, and cannot absorb moisture effectively. If the drought suddenly ends with heavy rainfall, the rain cannot infiltrate the soil and flows over the surface as a flood.
Most climate models predict that drought will increase in the future. The Intergovernmental Panel on Climate Change’s Fifth Assessment Report concluded that a reduction in streamflow (the amount of water flowing in rivers, which is one way of measuring drought) would occur in large parts of the Americas, Europe and Oceania, as shown on the map below.
- The percentage change of mean annual streamflow if temperatures rise 2°C above the the average temperature of the period 1980-2010. Red colours indicate a strong decrease, while blue colour represent a strong increase. The strength of the colour shows how strongly different studies agree; for example, very dark blue means that almost all the studies agreed that there would be a major increase in streamflow. Source: Schewe et al, 2013.
The increase in drought has most badly hit the Horn of Africa. According to Oxfam (2017), in the decade 2007-2017 there have been seven major droughts in East Africa and ten of the past sixteen long rains were dry compared to average. The graphs below show the temperatures in East Africa and that they have been rising quickly in recent years.
- Rising temperatures in East Africa. Source: Oxfam, 2017.
However, there is a contradiction between forecasts and actual data in this case. In the IPCC’s map of streamflow predictions (Schewe, 2013; see above), East Africa was forecast to have an increase in water. The graph below shows the actual variation in rainfall (left line) and the predicted variation (right line). This is known as the ‘East African Paradox’. Rowell and Booth (2015) assessed the reasons why there is so much disagreement, and found that it was likely to be a problem with the modelling of such a complex issue – climate models take in huge amounts of data and it is possible that the models may be wrong. This is very important because it suggests that for some areas of the world, droughts may in fact be more likely than expected.
- The East African Climate Paradox: Despite predictions suggesting the region should be wetter, it is actually experiencing more droughts. Source: Rowell and Booth, 2015, in Oxfam, 2017.
Is the weather really getting more extreme?
There are some reasons to suspect that the increase in extreme weather can partly be explained by the fact that we know about more extreme weather. We have better technology to measure the weather, and better historical records of what the weather did in the past thanks to improvements in techniques such as dendrochronology, sediment sampling and ice core measurements.
There are also better communications between remote parts of the world, so a major tropical storm is now reported better. For example, the World Meteorological Organisation maintains the Severe Weather Information Centre (HKO, 2018) which provides real-time information on global severe weather to the media and public. In Europe, the Network of European Meteorological Services provides a similar service called ‘MeteoAlarm’ including forecasting and gradation of severity warnings (EUMETNET, 2018).
These all combine to mean that we have a better idea of what is going on in the world. However, as the evidence above shows, there are more extreme events even in places that have a long history of good recording.
Bureau of Meteorology (2018). Observed changes in our climate system
http://www.environment.gov.au/climate-change/climate-science-data/climate-science/understanding-climate-change/indicators and http://www.bom.gov.au/state-of-the-climate/australias-changing-climate.shtml Accessed 31 March 2018
EPA, n.d.a. Climate Change Indicators: Weather and Climate. https://www.epa.gov/climate-indicators/weather-climate
EPA, n.d.b. Climate Change Indicators: Tropical Cyclone Activity. https://www.epa.gov/climate-indicators/climate-change-indicators-tropical-cyclone-activity
EUMETNET [Network of European Meteorological Services], 2018. No title. http://www.meteoalarm.eu/en_UK/0/0/EU-Europe.html
Fischer and Knutti, 2015. Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nature Climate Change, volume 5, pages 560–564 (2015). https://www.nature.com/articles/nclimate2617 With graphics sourced from Mathiesen, 2015. Extreme weather already on increase due to climate change, study finds.
Graham, 2000. Drought: The Creeping Disaster. https://earthobservatory.nasa.gov/Features/DroughtFacts/
HKO [Hong Kong Observatory], n.d. Severe Weather Information Centre. http://severe.worldweather.org/
Oxfam, 2017. Oxfam Media Briefing: A Climate In Crisis – How Climate Change Is Making Drought And Humanitarian Disaster Worse In East Africa. https://www.oxfam.org/sites/www.oxfam.org/files/mb-climate-crisis-east-africa-drought-270417-en.pdf
Pidcock and Pearce, 2017. Mapped: How climate change affects extreme weather around the world. https://www.carbonbrief.org/mapped-how-climate-change-affects-extreme-weather-around-the-world
Rowell and Booth, 2015. Reconciling Past and Future Rainfall Trends over East Africa. Met Office Hadley Centre. https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-15-0140.1#
Schewe et al., 2013. Percentage change of mean annual streamflow for a global mean temperature rise of 2°C above 1980–2010. In IPCC (2014) Fifth Assessment Report. https://www.ipcc.ch/pdf/assessment-report/ar5/wg2/WGIIAR5-Chap4_FINAL.pdf via https://www.ipcc.ch/report/ar5/wg2/
Environmental impacts of climate change: Weather: Learning activities
- Distinguish between weather and climate? 
- Give two examples of weather events, and two examples of climate events. 
- Define ‘incidence’ in relation to weather events. 
- Define ‘severity’ in relation to weather events. 
- Suggest why it is hard to determine whether a single weather event was due to climate change. 
- Describe the changes in extreme heat in Australia in both time and space. 
- Describe the changes in hurricanes in the North Atlantic in both time and space. 
- Define ‘drought’. 
- Explain how drought can lead to flooding. 
- Describe the expected changes in the global spatial variation of drought. 
- What is the ‘East African Drought Paradox’ and what factors might cause it? 
Use the Severe Weather Information Centre at http://severe.worldweather.org/ to find the nearest severe weather event to your location. Describe the severity of the event, and look for meteorological records to suggest whether the incidence and severity is changing over time for this type of weather event.
The map above shows 144 extreme weather events across the globe for which scientists have carried out attribution studies. The different symbols show the type of extreme weather; for example, a heatwave, flood or drought. The colours tell you whether or not the attribution study found climate change had played a role in that event (see the key on the right-hand side).
What does this map suggest about the future of extreme weather?
© Matthew Burdett, 2018. All rights reserved.
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