The world's major climate zones — polar, temperate and tropical — are transforming as we watch

Beyond the receding glaciers and disappearing coastlines, several recent studies have taken a big picture view of how climate change is affecting the globe's major climate zones and has brought about alarming transformations.

Climate zones — like tropical, temperate or polar — represent more than just temperature; they represent water resources, vegetation, animal life and even where and how humans can live.

"Often in our community, we say climate is what you expect and weather is what you get. And in this case, it's almost like we don't really know what to expect anymore," climate researcher Laura Landrum told Quirks & Quarks producer Amanda Buckiewicz.

Over the years, scientists have painted a stark picture of how climate change is altering the world's biomes in a fundamental way. A global 2018 study published in the journal Science showed that every ecosystem on Earth could face a dramatic shift if climate change continues unabated. Now scientists are reporting those shifts are beginning to happen — and showing how much worse it could get if it goes unchecked.

The Arctic is transitioning to a new climate

A recent study published in the journal Nature says that the Arctic is beginning to shift into a new climate regime.

"The Arctic climate is changing very rapidly and extremely, so it's in a different climate, meaning that the statistics of it now are very different than they were in the 20th century," said Landrum, the lead author of the paper.

Using decades of observations in the Arctic, combined with models and computer simulations, Landrum was able to statistically define the boundaries of a supposedly "normal" Arctic climate. And this showed that current conditions are well beyond normal.

They looked at indicators like sea ice cover, surface temperature and days of rain, and found that the year-to-year fluctuations have now moved completely outside the bounds of natural variability. 

The researchers then projected the trajectory of these shifts out to the year 2100. They found that a new, largely unrecognizable Arctic will have emerged by then. It will be completely ice-free for three to four months of the year; fall and winter temperatures could be 16 C to 28 C warmer than they are now; and the rainy season would be extended by two to four months each year.

A climate regime like this has never seen before in human evolutionary history. 

"It usually takes thousands of years to make these changes, and it certainly has not warmed up on a planetary scale over the last several thousand years, as warm as we're seeing now," said Landrum.

The Tropics are expanding

Another study looked at how the Earth's tropics are expanding — and found that the ocean is to blame.

The tropics wrap around the globe closest to the equator and get the most direct sunlight throughout the year. The interior of the tropics is hot and lush, and home to the world's rainforests. However, the edges of the tropics — the subtropical region — is hot and dry, and is where many of the world's deserts can be found.

A 2006 study in the journal Science found that the tropics were expanding toward the poles, causing those dry, arid regions to spread. This has caused the paths of tropical storms to shift, and led to more severe droughts — and seasonal wildfires — around the Mediterranean and in California and Australia.

"Under an expanding tropic, the Sahara-like, desert-like climate will move toward the North, then the Mediterranean countries will have a serious problem of water resources. Portugal, Italy, Spain, Greece and Turkey will suffer more drought in the future," said Hu Yang, a climate scientist at the Alfred Wegner Institute.

Yang recently led a study looking at whether ocean circulation could be the cause of this expansion. 

By analyzing water temperatures in the major ocean currents as well as satellite observations of sea surface temperature, they discovered that excess heat build up in the ocean is what is driving the expansion of the tropics.

"When we found the answer of expanding topics, of course, we are very excited because we worked on this topic for more than five years," said Yang. "But on the other hand, we're also very sad because our result shows that expanding tropics are not some natural climate variability. It is really climate change."

Amazon rainforest could shift to a savannah

Another study found that shifting rainfall patterns in the tropics could see large parts of the Amazon rainforest shift into a drier, savannah-like ecosystem. 

"What we know is that the rainforest needs certain rainfall amounts to be sustained. And what happens with climate change is that the rainfall in Amazon decreases so much that you don't have a lot of areas with rainfall — about 2,000 millimetres — anymore, so you don't have a large zone with stable rainforest," said Lan Wang-Erlandsson, an author on the study.

The study, which was produced by the Stockholm Resilience Centre, looked at current rainfall levels, and found that they could support either a savannah or a rainforest ecosystem. But the team also looked ahead to the end of the century, and found that if greenhouse gas emissions remain at their current rate and rainfall levels continue to fall, by the end of the century the Amazon rainforest could shrink by 66 per cent, as vast swaths shift into open forest and grassland. 

Wang-Erlandsson cautions that while these results may seem catastrophic, they're not set in stone yet.

"What we show is this scenario of severe climate change. We don't have to head toward this severe climate scenario," she said.

"We can see where we could be heading if we don't do anything; that is what is at stake. That's a cost of inaction, but also that we still have that space for action, I think."

Written and produced by Amanda Buckiewicz.