Rainfall rates exceeded 20 mm/hour for more than six hours across multiple high-elevation rain gauges in these regions, with the Kaweka, Maungaharuru, Raukumara Ranges of northern Te Matau-a-Māui witnessing particularly severe rainfall totals.

Some of the worst affected areas from this extreme rainfall event were communities with lifeline infrastructure (roads, bridges, water supplies) exposed to fluvial flooding at multiple locations. 225,000 homes were without energy during the peak of the storm, while the government has described Cyclone Gabrielle as a multi-billion dollar event with impacts comparable to the 2011 Waitaha  (Canterbury) earthquakes.

To analyse whether and to what extent human-caused climate change altered the likelihood and intensity of this extreme rainfall, scientists from Aotearoa New Zealand, the Netherlands, Germany, the US and the UK used published, peer-reviewed methods to perform an event attribution study, focussing on the heavy rainfall associated with the most severe damages in Te Matau-a-Māui and Tairāwhiti, on the east coast of Te Ika-a-Māui, bounded by the Kaimanawa and Raukūmara mountain ranges on the west, and the south Pacific Ocean on the east, comparing the peak 48-hour rainfall during February 2023 with the highest 48-hour rainfall totals in past years.

Fig. 1: 2-day accumulated rainfall  from Cyclone Gabrielle  in the MSWEP data. The red highlight shows the study region- Te Matau-a-Māui/Te Tairāwhiti, on the east coast of Aotearoa New Zealand’s Te Ika-a-Māui.

Main findings

• The main damages in the study region (Te Matau-a-Māui and Te Tairāwhiti) from cyclone Gabrielle occurred as a consequence of the extreme rain that fell over two days while the cyclone stalled.
• Cyclone Gabrielle’s track and the potential for extreme rainfall and winds were well forecast with up to a week of lead time, allowing emergency services and the local population time to prepare. Preceding flooding over Tāmaki Makaurau (Auckland) two weeks earlier had raised awareness of the potential severity and impact of rainfall, and improved preparedness based on warnings. At the same time the antecedent rainfall likely also increased the likelihood of landslides.
• The mountainous nature of the study region means that most of the population lives in flood-prone valleys and coastal plains, and transport and communication infrastructure crosses exposed terrain.
• In individual locations across the region, events like these are rare, with return times ranging from 70 to 320 years. Averaged over the whole study area the return period is between 10 and 90 years.
• First, using the relationship between historical weather station data (1979-2023) and global mean temperature to extrapolate back to colder climates, we found that the 2-day maximum rainfall over Te Matau-a-Māui/Te Tairāwhiti region is now about 30% more intense than it might have been had human greenhouse gas emissions not warmed the climate by 1.2°C. This also means a rainfall event of this magnitude is now about four times more likely to happen than it was when the world was 1.2°C cooler than it is today.
• The increase in the observed records show what is expected from basic physics for heavy rainfall in a warming world. However, there are large uncertainties in these estimates due to the short period covered by their data and high variability in the region.
• To determine the role of human-induced climate change in these observed changes we also looked at the trends in climate models. The region is smaller than in most comparable attribution studies for climate models thus limiting the number of models with off-the-shelf data available that can plausibly simulate this type of event.
• The models that pass our evaluation test generally show a much smaller change in likelihood and intensity of extreme rainfall than we found in the historical weather station data. This discrepancy suggests that processes not captured by our model evaluation could play an important role. This means we cannot quantify the overall role of human-induced climate change.
• Looking at the future, for a climate 2°C warmer than in preindustrial times, models suggest that rainfall intensity will slightly increase, although the uncertainty remains large.
• The disagreement between model results and observations prevents us from concluding with certainty that human-induced climate change is the main driver making this event more likely. However, while multiple reasons could explain the absence of a trend in our model results, we have no alternative explanation for a trend in observations other than the expectation of heavy rainfall increasing in a warmer climate.
• It is therefore important to urgently reduce the exposure and vulnerability of communities to future flooding, particularly ensuring that lifeline infrastructure remains intact so communities can receive flood warnings and respond accordingly.

Key findings

The key findings of the analysis of the 2019/20 fire season in southeastern Australia are:

Fire Weather Index

• We consider the highest weekly mean Fire Weather Index (FWI) of the fire season for each grid point as a measure of the most intense fire risk, and the Monthly Severity Rating as a measure of the overall seasonal fire risk. These are averaged over the area of most intense bushfires, between the mountains (the Great Dividing Range) and sea in New South Wales (including the Australian Capital Territory) and Victoria.
• Four climate models for which FWI could be calculated show that the probability of a Fire Weather Index this high has increased by at least 30% since 1900 as a result of anthropogenic climate change. As the trend in extreme heat is one of the main factors behind this increase and the models underestimate the observed trend in heat, the real increase could be much higher. This is also reflected by a larger trend in the Fire Weather Index in the observations.
• The observed index in 2019 was exceptional with a probability of about 3% in any given year in the current climate to occur. FWI shows a significant trend towards higher fire weather risk since 1979. Compared with the climate of 1900, the probability of Fire Weather Index as high as in 2019/20 has increased by more than a factor of four. For the Monthly Severity Rating the probability has increased by more than a factor of nine. We can attribute part of this trend to climate change.
• The Monthly Severity Index increased by a factor of two in the models, compared with 1900, but this is not significantly different from no change. Again, the real increase could be higher.
• Projected into the future, the models simulate that a Fire Weather Index at the 2019/20 level would be at least four times more likely with a 2 ºC temperature rise, compared with 1900. Due to the model limitations described above this is likely an underestimate.

Heat

• We analyse the highest 7-day mean maximum temperatures of the year averaged over the same region as the Fire Weather Index.
• Observations show that a heatwave as extreme as observed in 2019/20 would have been 1 to 2 ºC cooler at the beginning of the 20^th century. Similarly, a heatwave of this intensity is about 10 times more likely now than it would have been around 1900.
• While all eight climate models that were investigated simulate increasing temperature trends, they all have some limitations for simulating heat extremes: the variability is in general too high and the trend in these heat extremes is only 1 ºC, substantially lower than observed. We can therefore only conclude that anthropogenic climate change has made a hot week like the one in December 2019 more likely by at least a factor of two. Given the larger trend in observations, we suspect that climate models underestimate the trend in extreme heat due to climate change. Coupled with the models’ tendency to overestimate variability, the increase in the likelihood of such an event to occur is likely much higher than the models suggest.

Drought

• We consider annual mean low precipitation and the driest month of each year’s fire season September-February, both averaged over the same region.
• Observations show non-significant trends towards more dry periods like the record 2019 annual mean and a non-significant trend towards fewer fire season dry months like December 2019.
• All ten climate models we considered simulate the statistical properties of the observations well. Collectively they show no trend in dry extremes of annual mean precipitation nor in the driest month of the fire season (September–February). We conclude that there is no attributable trend in dry extremes like the ones observed in 2019.

2019 conditions

• These attributions are for events as defined above at the threshold, or above, set by the 2019/20 fire season and thus assessing the overall effect of human-induced climate change. In individual years, natural variability in large scale climate system drivers can have a higher influence on fire risk than the trend.
• Besides the influence of anthropogenic climate change, the particular 2019 event was made much more severe by a record excursion of the Indian Ocean Dipole and a very strong anomaly of the Southern Annular Mode, which together explain more than half of the amplitude of the drought in the second half of 2019.

Full study

• See the full study at NHESS: Attribution of the Australian bushfire risk to anthropogenic climate change

There were three distinct heatwaves in southeast Australia during January and February, with the highest temperatures recorded from February 9th to the 12th. For much of the country, the heat peaked on the weekend of February 11th and 12th, when many places hit upwards of 113°F (45°C). The 2016-2017 heatwaves broke long-standing records in central New South Wales that were originally set back in January of 1939 (Figure 2).

The WWA team and colleagues from the University of New South Wales conducted a rapid attribution analysis to see how climate change factored into the exceptionally warm summer (December to February) of 2016-2017. The team also looked at the hottest three-day average February temperatures in Canberra and Sydney.

Regional level: New South Wales

The New South Wales record hot summer can be linked directly to climate change. Two different methods were used to reach this conclusion. First, drawing from a previously published analysis using coupled model simulations, we see that average summer temperatures like those seen during 2016-2017 are now at least 50 times more likely in the current climate than in the past, before global warming began. The team also performed an analysis based on the observational series from ACORN-SAT. This approach is similar to previous analyses used for record heat in the Arctic in 2016 and Central England in 2014. Comparing the likelihood of this record in the climate of today compared with the climate of around 1910 (before global warming had a big impact on our climate system and when reliable observations are available), the team again found at least a 50-fold increase in the likelihood of this hot summer.

The team then looked at the maximum summer temperature for New South Wales (see graphic below). Based on climate model simulations (weather@home and CMIP5) and observational data analysis (ACORN-SAT), maximum summer temperatures like those seen during 2016-2017 are now at least 10 times more likely in the current climate than in the past, before global warming began. In the past, a summer as hot as 2016-2017 was a roughly 1 in 500-year event. Today, climate change has increased the odds to roughly 1 in 50 years — a 10-fold increase in frequency. In the future, a summer as hot as this past summer in New South Wales is likely to happen roughly once every five years. In addition, climate change has increased the intensity of an exceptionally hot summer like this by roughly 1ºC (1.8°F). In the future, the intensity increases by roughly 2°C (3.6°F).

Local level: Canberra and Sydney heatwaves

The team also looked at the local scale to see if a climate change role could be measured in the heat waves that hit Canberra (population ~380,000) and Sydney (population ~4.9 million). Climate has much larger variability at the city level compared to a big area like New South Wales. This can make it more difficult to see the influence of climate change within the overall noise of the weather system.

In Canberra, temperatures hit 96.8oF (36°C) on February 9th and 104oF (40oC) on both February 10th and 11th. Using the weather@home model and ACORN-SAT observations, we analyzed three-day average maximum temperature. Both the observational data and the climate model simulations show that climate change increased the likelihood of the kind of extreme three-day heat observed in Canberra. The weather@home results point to at least a 50 percent increase in the chance of a heatwave like that.

For Sydney, a coastal city, the effect of climate change on this heat wave is less clear. Observations show that climate change increased the chance of such a heat wave occurring, but the high year-to-year variability makes identifying a clear human influence more difficult.

The future

The heat seen this past summer across parts of Australia is still rare in our current climate. However, if greenhouse gas emissions are not dramatically reduced, intense summer heat will become the norm in the future.

There is indisputable evidence that climate change is harming the reef. Yet, so far, no one has assessed how much climate change might be contributing to bleaching events such as the one we have just witnessed.

Unusually warm sea surface temperatures are strongly associated with bleaching. Because climate models can simulate these warm sea surface temperatures, we can investigate how climate change is altering extreme warm conditions across the region.

Daily sea surface temperature anomalies in March 2016 show unusual warmth around much of Australia. Author provided using OSSTIA data from UK Met Office Hadley Centre.

We examined the Coral Sea region (shown above) to look at how climate change is altering sea surface temperatures in an area that is experiencing recurring coral bleaching. This area has recorded a big increase in temperatures over the past century, with March 2016 being the warmest on record.

Examining the human influence

To find out how climate change is changing the likelihood of coral bleaching, we can look at how warming has affected the likelihood of extremely hot March sea temperature records. To do so, we use climate model simulations with and without human influences included.
If we see more very hot March months in simulations with a human influence, then we can say that climate change is having an effect, and we can attribute that change to the human impact on the climate.
This method is similar to analyses we have done for land regions, such as our investigations of recent Australian weather extremes.
We found that climate change has dramatically increased the likelihood of very hot March months like that of 2016 in the Coral Sea. We estimate that there is at least a 175 times increase in likelihood of hot March months because of the human influence on the climate.

The decaying El Niño event may also have affected the likelihood of bleaching events. However, we found no substantial influence for the Coral Sea region as a whole. Sea surface temperatures in the Coral Sea can be warmer than normal for different reasons, including changes in ocean currents (often related to La Niña events) and increased sunshine duration (generally associated with El Niño conditions).

Overall, this means that the influence of El Niño on the Coral Sea as a whole is weak. There have been severe bleaching events in past El Niño, neutral and La Niña years.

We estimate that climate change has increased temperatures in the hottest March months by just over 1℃. As the effects of climate change worsen we would expect this warming effect to increase, as has been pointed out elsewhere.

March 2016 was clearly extreme in the observed weather record, but using climate models we estimate that by 2034 temperature anomalies like March 2016 will be normal. Thereafter events like March 2016 will be cooler than average.

Overall, we’re observing rapid warming in the Coral Sea region that can only be understood if we include human influences. The human effect on the region through climate change is clear and it is strengthening. Surface temperatures like those in March 2016 would be extremely unlikely to occur in a world without humans.

As the seas warm because of our effect on the climate, bleaching events in the Great Barrier Reef and other areas within the Coral Sea are likely to become more frequent and more devastating.

Action on climate change may reduce the likelihood of future bleaching events, although not for a few decades as we have already built in warming through our recent greenhouse gas emissions.

Find out about the methods we used.