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Environment

Earth Stopped Getting Greener 20 Years Ago

By Chelsea Harvey

August 15, 2019—The world is gradually becoming less green, scientists have found. Plant growth is declining all over the planet, and new research links the phenomenon to decreasing moisture in the air—a consequence of climate change.

The study published yesterday in Science Advances points to satellite observations that revealed expanding vegetation worldwide during much of the 1980s and 1990s. But then, about 20 years ago, the trend stopped.

Since then, more than half of the world’s vegetated landscapes have been experiencing a “browning” trend, or decrease in plant growth, according to the authors.

Climate records suggest the declines are associated with a metric known as vapor pressure deficit—that’s the difference between the amount of moisture the air actually holds versus the maximum amount of moisture it could be holding. A high deficit is sometimes referred to as an atmospheric drought.

Since the late 1990s, more than half of the world’s vegetated landscapes have experienced a growing deficit, or drying pattern.

Climate models indicate that vapor pressure deficit is likely to continue increasing as the world warms—a pattern that “might have a substantially negative impact on vegetation,” the authors write.

It’s not the first study to document the global decline in vegetation. A 2010 study in Science was among the first to demonstrate that the greening increases of the 1990s had stalled or reversed. That study also suggested that the declines were probably water-related.

That’s not to say every last corner of Earth is losing its vegetation. Some recent studies have revealed that parts of the Arctic are “greening” as the chilly landscape warms. And there’s increasing plant growth still happening in other regions of the world, as well.

But on a global scale, averaged across the entire planet, the trend is pointing downward.

The declines challenge an argument often presented by skeptics of mainstream climate science to downplay the consequences of global warming: the idea that plants will grow faster with larger amounts of carbon dioxide. The argument hinges on the idea that food supplies will increase.

It’s largely a red herring, as climate scientists have patiently explained for years. Rising CO2 does benefit plants, at least up to a point, but it’s just one factor. Plants are also affected by many other symptoms of climate change, including rising temperatures, changing weather patterns, shifts in water availability and so on.

Many researchers have suggested that climate change, on the whole, is likely to be a net negative for much of the world’s vegetation, including agricultural crops. The new study would seem to suggest that those consequences are already in motion.

And as climate change affects plant growth, declining plant growth may also affect the pace of climate change.

Just last week, an anticipated report from the Intergovernmental Panel on Climate Change emphasized the importance of land and vegetation as climate mitigation tools (Climatewire, August 8, 2019). Forests and other vegetated landscapes tend to be significant carbon sinks, sucking carbon dioxide out of the atmosphere and storing it away. Less growth, on the other hand, means less carbon storage.

Atmospheric moisture, like carbon dioxide, is just one factor among many that may affect the world’s vegetation in the coming years. But since the drying trends seem to have had a particularly significant impact over the last two decades, the authors suggest that it “must be examined carefully when evaluating future carbon cycle responses.”

Scientific American, August 15, 2019

https://www.scientificamerican.com/article/earth-stopped-getting-greener-20-years-ago/