Earth's accelerating plant growth

For decades, scientists have been trying to figure out what all the carbon dioxide we’ve been putting into the atmosphere has been doing to plants. It turns out that the best place to find an answer is where no plants can survive: the icy wastes of Antarctica. As ice forms in Antarctica, it traps air bubbles. For thousands of years, they have preserved samples of the atmosphere. The levels of one chemical in that mix reveal the global growth of plants at any point in that history. “It’s the whole Earth — it’s every plant,” said J Elliott Campbell of the University of California, Merced, USA.

Analysing the ice, Elliott and his colleagues have discovered that in the past century, plants have been growing at a rate far faster than at any other time in the past 54,000 years. Writing in the journal Nature, they report that plants are converting 31% more carbon dioxide into organic matter than they were before the Industrial Revolution. The increase is due to the carbon dioxide that humans are putting into the atmosphere, which fertilises the plants, Elliott said.

The carbon in the extra plant growth amounts to a staggering 28 billion tonnes each year. For a sense of scale, that’s three times the carbon stored in all the crops harvested across the planet every year. “It’s tempting to think of photosynthesis at the scale of the entire planet as too large to be influenced by human actions,” said Christopher B Field, the director of the Stanford Woods Institute for the Environment, USA, who was not involved in the study. “But the story here is clear. This study is a real tour de force.”

Starting from the Industrial Revolution, humans began to pump carbon dioxide into the atmosphere at a prodigious rate. Since 1850, the concentration of the gas has increased over 40%. Since plants
depend on carbon dioxide to grow, scientists have long wondered if that extra gas might fertilise them.

The question has been hard to answer with much certainty.

Real-world measurements
For one thing, a plant relies on more than just carbon dioxide. It also needs water, nitrogen and other compounds. Even with a perfect balance of nutrients, plants may grow at different rates depending on the temperature. To get some real-world measurements of plant growth, some scientists have built enclosures so that they can determine the precise amounts of carbon dioxide as well as the growth of plants. They can even run experiments by flooding the enclosures with extra carbon dioxide.

Trees and other plants in these enclosures have indeed grown faster with more carbon dioxide. But it’s been hard to extend these results to the planet as a whole. Scientists found that plants responded differently to carbon dioxide in different parts of the world. The logistical challenge of these experiments has mostly limited them to Europe and the United States, leaving huge swaths of forests in the tropics and the far north little studied.

More recently, scientists have turned to satellites to get clues to what plants have been doing. They have measured how green the land is, and from that data they have estimated the area covered by leaves. But this method has its shortcomings, too. Satellites cannot see leaves hiding under clouds, for example. And the size of leaves serves as only a rough guide to a plant’s growth. If a plant builds bigger roots, that growth will be hidden underground.

In the mid-2000s, atmospheric scientists discovered a powerful new way to measure plant growth: by studying an unimaginably rare molecule called carbonyl sulfide. Carbonyl sulfide — a molecule made of a carbon atom, a sulphur atom and an oxygen atom — is present only in a few hundred parts per trillion in the atmosphere. That is about a million times lower than the concentration of carbon dioxide. Decaying organic matter in the ocean produces carbonyl sulfide, a gas that then floats into the atmosphere.

Plants draw in carbonyl sulfide along with carbon dioxide. As soon as it enters their tissues, they destroy it. As a result, the level of carbonyl sulfide in the air drops as plants grow. “You can see it in real time,” said Max Berkelhammer, an atmospheric scientist at the University of Illinois at Chicago, USA. “In the morning when the sun rises, they start to pull it out.” This discovery led scientists to go to Antarctica.

The air that reaches the South Pole is so well mixed that its carbonyl sulfide level reflects the worldwide growth of plants.

Historical record
As ice forms in Antarctica, it captures bubbles of air, creating a historical record of the atmosphere reaching back thousands of years. Last year, Elliott and his colleagues analysed carbonyl sulfide records from the past 54,000 years. Over the course of several thousand years at the end of the ice age, the gas dropped significantly.

Elliott said the decline reflected the retreat of the glaciers. As new land was uncovered, plants sprang up and began destroying carbonyl sulfide. It’s more challenging to interpret the more recent record in the ice. Since the Industrial Revolution, humans have added extra carbonyl sulfide through textile manufacturing and other activities. This infusion of carbonyl sulfide has raised levels of the gas in the ice over the past century.

But Elliott and his colleagues found that it hasn’t increased very much. As we have been adding carbonyl sulfide to the atmosphere, plants have been pulling it out. In fact, the scientists found, they have been pulling it out at a staggering rate. “The pace of change in photosynthesis is unprecedented in the 54,000-year record,” Elliott said. While photosynthesis increased at the end of the ice age, he said, the current rate is 136 times as fast. With all that extra carbon dioxide going into plants, there has been less in the air to contribute to global warming. The planet has warmed 1.4 degrees Fahrenheit since 1880, but it might be even hotter if not for the greening of the Earth.

Max, who was not involved in the new study, said the research would serve as a benchmark for climate projections. “It means we can build more accurate models,” he said.