New study reveals global warming is affecting night-time temperatures differently

From the poles to the tropics, the oceans to our cities, we've mapped the fluctuations in temperature that are leading to a climate crisis.

But strangely, little attention has been paid to the world's circadian landscapes of night and day. And a new study shows our nocturnal environment is actually warming at a faster rate than our daytime surrounds - and it could prove too much for many species.

After analysing more than three decades of daily temperature data from all over the world, researchers from the University of Exeter have concluded there's an asymmetry in our planet's warming as it rotates on its axis.

The climate records spanned from 1983 to 2017, providing the team with a hefty database of six-hourly surface temperature readings covering virtually the entire planet during some of the warmest years in recorded history.  

In some spots, the days warmed considerably while night time temperatures barely budged. There were even times of considerable cooling for some environments.

But the bigger picture was surprising. Across more than half of the planet's land surface, the average annual temperature rise at night was a quarter of a degree Celsius more than that of the day's.

A fraction of a degree each year might sound tiny, but over time these increments of heat could add up to have a significant effect on the ecology.

"Species that are only active at night or during the day will be particularly affected," says ecologist and lead author, Daniel Cox from the University of Exeter.

To get a better understanding of the environmental forces at work, the team also collected a bunch of data on other related climate factors such as humidity and precipitation.

They also compared regional differences in vegetation growth.

Putting it together, something as simple as more cloud cover could easily account for the imbalance in heating.

Global warming traps extra amounts of energy close to the planet's surface, encouraging the atmosphere to hold moisture which then condenses into clouds. There's no real secret there.

We also know clouds do a great job of reflecting certain wavelengths of light, either away into space or back down to the ground.

During the day, this can help shield the surface from the full blast of sunlight, keeping a bit of a lid on temperatures. Without this shading effect, we could expect our planet's surface to roast.

At night, the process is reversed. Heat radiating from the ground has a harder time making it into space, keeping the surface a touch warmer.

We've all felt the chill of a cloudless winter night to understand the basic mechanism, but having hard data can help us better model what we might expect in coming years.

Putting aside variations in temperature change across different locations through time, changes in temperature variation between night and day could have a profound impact on rainfall, which in turn determines how well plants grow.

Even with a general increase in rainfall, extra cloud cover during the day risks reducing the amount of light plants need to photosynthesise.

"Warming asymmetry has potentially significant implications for the natural world," says Cox.

"We demonstrate that greater night-time warming is associated with the climate becoming wetter, and this has been shown to have important consequences for plant growth and how species, such as insects and mammals, interact."

Understanding the full implications of daily fluctuating temperatures and cloud formation is going to take a lot more research.

Clouds can be surprisingly complex phenomena, especially when we take into account greenhouse gases, the influence of dust levels, and even less Earth-bound variables.

Knowing how much they'll help or hinder our efforts to limit rising temperatures isn't a simple question to answer.

This research was published in Global Change Biology.

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