Explainer: Why does a warming climate sometimes make things wetter?

We know the climate is warming - 2021 was the sixth-warmest year on record and global temperatures have increased approximately 1.1C since 1850-1900 - so, as might be expected, often the impact of the higher temperature is drier weather.

Europe bore the brunt of this in summer 2022, with scorching heat waves, relentless drought and horrific wildfires.

But here in Aotearoa we've just had our warmest and wettest winter on record, with a series of extreme rainfall events, including the disastrous storms in August that led to flooding and slips in Nelson, Tasman, the West Coast and Marlborough.

Looking ahead to the future, along with an increase in severity and frequency of droughts, climate change projections for New Zealand include an increase in extreme rainfall events and an increase in rainfall in general (mean average precipitation) for those areas which are already our wettest, such as the west coast of the South Island.

So, what's the connection between a warming climate and wetter weather?

There are many factors at play. But two key ways that scientists believe climate change could lead to wetter weather are its impacts on the water cycle and the jet streams.

In its simplest form, the water cycle is the continuous movement of water around the Earth and atmosphere, through evaporation from the ocean and waterways into the atmosphere as water vapour, condensation into clouds, precipitation back to the surface of the Earth as rain or snow.

With greater heat, evaporation increases. Alongside this, according to the laws of thermodynamics, a warmer atmosphere can hold more moisture and more energy - for every degree of warmth, there's capacity for an extra 7% more water vapour.

When conditions come together to squeeze out the water, more rain comes back down, with more intensity.

It's thought that climate change could impact the jet streams - the wavy currents of fast-moving air blowing west to east around the planet.

Earth has four main jet streams - two polar and two subtropical - which are formed when cold air from the poles meets with hot air from the tropics. They're big players in global weather systems, carrying storms across continents and impacting temperature and rainfall.

The planet is not warming at the same rate - the poles are heating up faster than the equator, particularly the Arctic. Some experts believe these changes could disrupt the jet streams and push them towards the poles. There's evidence that this is already happening in both hemispheres.

A 2021 study published in the Proceedings of the National Academy of Sciences used Greenland ice-core records to reconstruct the past 1250 years of behaviour of the North Atlantic jet stream, looking at natural variations over time, and projected that it will migrate northward by 2060 if global warming is not curbed.

Down here in the southern hemisphere, Antarctica hasn't heated up as much as the Arctic, but there is evidence that the southern sub-tropical jet stream has begun to move south, coming closer to New Zealand. In combination with a warmer and wetter atmosphere, this can hit us with some intense rain.

Peter Gibson, NIWA climate scientist, explains this interplay between 'atmospheric rivers' (ARs) - long corridors of water vapour carried from the tropics which have the potential to dump very large amounts of rain - and the jet stream.

"Because ARs are carried along by the jet, they are inherently linked… There is evidence that the southern hemisphere westerly jet has already begun to shift slightly further south in recent decades and that in most places ARs deliver more precipitation in recent decades due to climate change.

"For NZ, this southward shift of the jet, combined with a warmer atmosphere, likely means that ARs have been able to deliver greater precipitation totals especially across southern NZ in recent decades."

There is also a theory that climate change may be making the northern jet stream wavier, leading to more extreme weather events, and making them stick around for longer. That's an alarming prospect, but NIWA principal climate and atmospherics scientist Olaf Morgenstern cautions against jumping to conclusions that is happening here too.

"For starters, the Antarctic has not warmed nearly as much as the Arctic. While that remains the case, I don’t think we can say that things are getting more erratic here."

Gibson says that more research is needed to understand the impacts of climate change on the southern jet streams and how that might impact NZ’s weather in the future.

"Because a warmer atmosphere can hold more water vapour, most parts of NZ are projected to see continued higher rainfall amounts during very extreme rainfall events under climate change. Other secondary changes in weather patterns, are currently less certain, like how the regional jet stream or blocking patterns may change in the future to enhance precipitation changes in certain regions across NZ… More research is needed to untangle these two processes for our region."

While debate and research continues on the ways climate change is impacting our weather, Morgenstern has no doubt that the effects are already being felt.

"It’s already happening. All regions of the planet are already seeing the impacts of climate change… the extremely wet, warm winter this year fits into this pattern."