After months of extreme heat and drought in Australia, the fires finally came. Australia experiences bushfires every year. But this year they are particularly extreme — and summer in the southern hemisphere has only just begun.

Eight million hectares have been destroyed by the flames so far. Twenty-five people and millions of animals have been killed. Entire regions have been left without power and clouds of smoke now cover half the continent, according to the preliminary assessment of the devastating fires.

But Australia isn't the only place which is burning. In 2019, online platform Global Forest Watch Fires (GFW Fires) counted over 4.5 million fires worldwide that were larger than one square kilometer. That's a total of 400,000 more fires than 2018.

"The number of fires and their size varies from year to year, but the big trend is that the risk of fire is increasing globally," Susanne Winter, the Forest Program Manager at WWF Germany, told DW.

The reasons fires start and take hold in the first place are complex. But experts are now pointing to a connection between the increasing risk of wildfires and warmer ocean temperatures as a result of climate change.

Read more: My Australian paradise lost 

Warmer seas act as fire accelerators

Manmade greenhouse gases have raised Earth's average temperature by an estimated one degree Celsius since the 19th century. The sea surface has also warmed by 0.8 degrees Celsius. The warmer the ocean gets, the less energy and CO2 the water is able to absorb and store from the atmosphere.

"[The ocean] is like air conditioning for the planet," explains Karen Wiltshire, the vice director of the Alfred Wegener Institute for Marine and Polar Research.

The consequences of this could be devastating. If the sea continues to warm, it will have an enormous impact on the climate from extreme temperatures, storms and droughts to floods and late rainy seasons which disrupt ecosystems.

When strong winds tear through hot and dry landscapes such as Australia, the bushfire risk increases significantly. But the risk is also growing in regions which were once temperate and cool.

Read more: Oceans play role in Australian bushfires drama, say experts

Even the Arctic is burning

In addition to the large fires burning in Europe and California, 2019 also saw wildfires in the Arctic. "The kind of fires we have never seen before," says Clare Nullis from the World Meteorological Organization (WMO). Alaska experienced   record-breaking temperatures of up to 32 degrees Celsius, creating conditions for the fires. According to the WMO, we can expect to see forests in the northern hemisphere burn like they never have over the past 10,000 years.

In Alberta in northern Canada, hundreds of fires burned for months over 800,000 hectares of land in summer 2019. According to estimates by Russian authorities, around 9 million hectares of forest were burned in Siberia — a bigger area than the entire country of Portugal. The toxic smoke settled over villages and towns.

Humans to blame for burning forests

Fires are actually a natural process in the regeneration and renewal of ecosystems. However, 96% of the world's fires are now either deliberately lit or unintentionally caused by humans. Only 4% of fires start naturally; for example due to lightning strikes, according to a report by WWF.

Many areas are cleared using the slash and burn method to make way for agriculture, livestock or industry, particularly in the Amazon region. In Indonesia as well, over 27 million hectares of forest have been destroyed since 1990 for paper and palm oil industries.

Africa

Data from the Global Forest Watch Fires shows that many fires are blazing across Africa, from South Sudan to West Africa. Experts say high population density has led to increasingly intensive use of natural resources, meaning the ecosystems have less and less time to recover. And the fires are also becoming more common.

"The main reason for this is the widespread use of shifting cultivation," explains Winter. "Landowners and farmers [use fire to clear] their fields to quickly get rid of vegetation and make the soil fertile in the short term." Some of these fires can get out of control, leading to larger wildfires.

The Amazon

There were more fires across South America last year than there have been since 2010. Large areas of forest were cleared for agriculture in the Amazon region in 2019. "These were not natural causes," says Nullis.

"The forest fires in Brazil are politically motivated," adds Winter. "Of course, we cannot compare them to the fires in Africa."

Between January and November 2019, more than 80% of forest was destroyed compared to the previous year. Thirty years ago, the Amazon was still so humid that fires like the ones we see today would not have been possible, says Winter. However, the Amazon is getting drier and drier thanks to more and more clearing of land.

Read more: Amazon wildfires set to cause irreversible damage

Climate change and the cycle of fire

Deforestation, climate change and the risk of wildfires are all directly linked.

"We are dealing with a feedback effect here," says Winter. "More deforestation means an increase in climate change, which increases the chances of the vegetation drying out, which in turn increases the risk of fire and so on."

And the fires continue to increase greenhouse gases in the atmosphere. According to Greenpeace, around 8 billion tons of CO2 are released by fire every year. This is about half as much as the emissions caused by the burning of coal around the world.

The bushfires in Australia have already released half the amount of CO2 that the continent would otherwise produce during typical year. And the smoke trails are now spreading across the Pacific to Argentina and Chile.

Editor's note: The original version of this article stated that there were two and a half times more fires in 2019 than in 2001. We later removed this figure after discovering an error. The lower number from 2001 reflected the fact that satellite data at the time was collected by NASA's Terra satellite. Starting in mid-2002, this data was expanded to include detections from the Aqua satellite, which caused the number of detections to rise significantly.