The moon appears to be having a small effect on Earth鈥檚 changing climate that could hasten the point at which the world鈥檚 1.5掳C climate target is breached.
The angle of the moon鈥檚 orbital plane relative to the equator changes in a predictable 18.6-year cycle, but it wasn鈥檛 known what impact this has on Earth鈥檚 surface temperatures.
In a modelling at the University of Reading, UK and his colleagues found that the cycle will have a modest cooling effect on global average surface temperatures this decade, followed by a slight warming one in the 2030s. The pattern could also partly explain a purported global warming slowdown in the 2000s.
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The researchers believe this cycle changes how the tides affect the mixing between warmer waters at the ocean鈥檚 surface and deeper, cooler waters, altering the rate at which the oceans can absorb heat.
鈥淚t鈥檚 a pretty small effect,鈥� says Hawkins. The estimated size of both the cooling and warming effect is about 0.04掳C. This is much smaller than the shift from the cooling weather pattern the world is currently experiencing to the warming El Ni帽o one, he notes.
鈥淸But it shows] that we do study the whole climate system, from the sun to the moon to the Earth. And we try and represent all of the factors we can think about, and how they might affect the climate,鈥� he says. While the moon鈥檚 impact may be small, Hawkins says it warrants inclusion in models used to project future climate scenarios if further research confirms the findings, which aren鈥檛 yet published in a peer-reviewed journal.
Without accounting for the moon鈥檚 influence, climate models predict that the world will exceed 1.5掳C of warming 鈥� a threshold nearly 200 countries aimed to avoid in the Paris Agreement聽鈥� between 2028 and 2033. Factoring in the lunar cycle narrows the predicted range to 2029 to 2032. The timing of the current cycle also means that in low emissions scenarios, the 1.5掳C threshold is likely to be met around a year earlier than previously thought.
The influence of the moon adds another explanation for why the Arctic is warming faster than the rest of the planet, due to the structure of the waters there, says Hawkins. The cooling phase of the lunar cycle also coincided with a period of global warming in the 2000s that was slower than models predicted at the time. Climate sceptics seized on the data to discredit global warming, although researchers subsequently found there was less of a slowdown than initially thought.
Hawkins says his study gives no ammunition for climate change deniers to blame the moon for global warming, as some have done with the sun. 鈥淚t鈥檚 a small effect; it goes up and down over a very predictable 18.6-year time scale," he says. "We'll see periods where it may be slightly faster, then may be slightly slower, but it averages out. And so it cannot explain any of the long term [warming] trends that we see.鈥�
at Imperial College London, who wasn't involved in the study, says the research shows the moon's effects are small and cyclical, and it is 鈥渆xtremely valuable鈥� to understand both human and natural influences on Earth鈥檚 temperatures. But he says the estimated impact on when 1.5掳C will be reached is so small as to be insignificant, as it will 鈥渄isappear in the noise of global [temperature] observations鈥�.
at the University of Leeds, UK, who also wasn't involved with the study, says the mechanism for how the moon is affecting global temperatures is 鈥渄efinitely plausible, but its importance is uncertain鈥�. He says the study is 鈥渘eat鈥� and could help future decadal projections of temperature rises.
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