Marlene Cimons writes for Climate Nexus, a nonprofit that aims to tell the climate story in innovative ways that raise awareness of, dispel misinformation about and showcase solutions to climate change and energy issues in the United States. She contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

One persistent assumption about the effects of climate change is that plants will thrive in warmer temperatures and an atmosphere of increasing carbon dioxide. But the reality turns out to be not so simple. In many parts of the world, just the opposite could occur — and with potentially disastrous results for billions of people who depend heavily on plants for food, fuel and jobs.

New research in the peer-reviewed journal PLOS Biology suggests plants in the north will remain limited by solar radiation — which is scarce at northern latitudes due to the shape of the Earth and its rotation, and is not likely to change as a result of climate — curbing any positive effects of warming and additional carbon dioxide. Furthermore, many plants in tropical regions will be unable to tolerate excessively high heat, especially if accompanied by drought. The result could be a loss in valuable growing days for populations who can least afford it and are ill equipped to cope with it.

"Those who think climate change will benefit plants need to see the light, literally and figuratively," said Camilo Mora, an ecologist and assistant professor in the geography department at the University of Hawaii at Mānoa and the study's lead author. "A narrow focus on the factors that influence plant growth has led to major underestimations of the potential impacts of climate change on plants, not only at higher latitudes but more severely in the tropics," he said. These impacts will expose "the world to dire consequences." [Animals and Plants Adapting to Climate Change]

Under these conditions, "choices become extremely limited for people who are already vulnerable," said Micah Fisher, a Ph.D. candidate at the university and a co-author of the study. "Without options in livelihoods or food security, impacts tend to disproportionally impact the poor."

The tropics could lose nearly one-third of their current plant-growing period if climate change continues unabated, which would have a significant impact on as many as 2.1 billion low-income people who rely on plants and their byproducts to survive, the study said.

The research shows that continued climate change will result in declines in plant-growing days by 2100, due to a combination of warming, drought and limited sun. Overall, the globe could see an estimated 11 percent reduction in the number of days with suitable conditions for plant growth, with some tropical regions facing the loss of up to 200 days per year by 2100.

"I think this is an important and novel insight, which highlights the synergistic consequences of a large and growing human population and its effect on the environment in which we live," said David Inouye, professor emeritus of biology at the University of Maryland, who studies the impact of climate change on the environment, but who was not involved in the new study. "Some human populations are certain to be losers as a consequence."

Global impact

The study provides a scale showing vulnerability for each of 194 countries. It maps the countries' exposure through the decrease of suitable growing days, their dependency on agriculture-related products and livelihoods, and their adaptive capacity through basic economic indicators.

The study also found that changes in suitable plant-growing days were negligible under strong and moderate mitigation scenarios, suggesting that even modest reductions in emissions could prevent such drastic changes and their associated consequences for ecosystems and people. Also, the researchers said there may be other ways to lessen the impact.

"For plants that we grow for food, fiber and fuel, we have a little more control, and there are some additional options — for example, switching to growing crop varieties that can thrive under warmer, drier conditions, or irrigating to minimize the effects of drought, although irrigation is already used extensively," said Iain Caldwell, a postdoctoral researcher at the university and another co-author of the study.

"Most of these options tend to be expensive, though," Caldwell added. "Since our research also shows that some of the most extreme changes in plant-growing conditions will occur in some of the poorest countries in the world, these options may require help from other, wealthier countries." Though these countries "may not necessarily be as heavily impacted, [they] are responsible for much of the emissions that have led to such climate change."

More carbon emissions don't mean more plants

Prior research that focused on temperature concluded that rising temperatures and carbon dioxide levels would be beneficial to agriculture. (A 1992 video financed by the coal industry, called "The Greening of Planet Earth," and its 1998 successor, "The Greening of Planet Earth Continues," made the same claim.) This became a talking point for those who failed to understand the huge uncertainties in the world's understanding of how plants and ecosystems will respond to climate change in the coming decades.

Plants, in fact, depend on the availability of water, appropriate temperatures, light and nutrients, "and the impact of changes in any of these resources can depend on how sensitive a species/ecosystem is, how quickly a species can adapt or evolve, and even how different species interact," said Ben Cook, a climate scientist at NASA Goddard Institute for Space Studies, who was not involved in the new research.

The assumptions that climate change was good for plants "probably originated in the idea that increased carbon dioxide, which plants use in photosynthesis, would stimulate productivity and plant growth, and that warmer temperatures would lead to longer growing seasons," Cook added. "Very quickly, however, it became apparent that — while true on a certain level — these are really gross generalizations and the ultimate response of plants and ecosystems is really quite a bit more complex."

The researchers who conducted the new study said it was not surprising that earlier studies looked at temperature, "since this is the most obvious indicator of a warming climate," Fisher said. "It is also difficult to conduct global modeling of climate change. But we are getting better. The models have come a long way and have developed a much more accurate predictive power. We tried to fit the elements of plant growth into what we believed was an innovative approach."

At locations where prior studies have been conducted, "plants are below their optimum temperature, and thus warming helps," Mora said, meaning there is enough warmth for some growth, but they would grow faster if it became warmer at these locations. "If you add carbon dioxide, they do even better," he said. "This would make sense to many, because as the planet gets warmer, those areas that are cooler, such as higher-latitude countries, will have better conditions for plant growth. Also, because we know that carbon dioxide is essentially food for plants, increases in carbon dioxide should result in more growth — if nothing else is limiting."

However, "the problem is for the rest of the world, where plants are already living in conditions close to or past their optimum for growth, additional warming could take plant species beyond thresholds for growth and, possibly, survival," Mora added.

Understanding the true complexity of climate

Overall, climate models are improving, "and we are continuing to develop better capabilities at understanding the nuances of climate change," Fisher said. "It speaks to the challenges of developing policies to address very real impacts arriving from very complex systems. That's part of the reason why people have missed the fact that these warming areas will not have the light to grow. It seems obvious, but we live in a complicated world, with a complex changing climate, and we have to find better forums to have these discussions."

Using satellite-derived data, the PLOS Biology study — which also included Jamie Caldwell and Brandon Genco, both of the University of Hawaii at Mānoa, and Steven W. Running, of the University of Montana school of forestry — identified the ranges of temperatures, soil moisture (water availability) and light (solar radiation) within which 95 percent of the world's plant growth occurs today. The researchers then used climate projections to count the number of days in a year that will fall within the suitable climate ranges for plant growth in the future. The researchers counted the number of suitable plant growing days for all terrestrial areas of the globe, but did not include areas covered in water.

"This is a nice study that really considers the aggregate change in climate conditions that will be relevant for vegetation growth in the future, instead of just single factors like temperature or drought," NASA's Cook said. "And because it's based on actual observations, it can potentially provide a useful benchmark for comparison of vegetation model simulations, which are more commonly used for these types of studies."

The study did find that warming trends will increase by 7 percent the number of days above freezing temperatures that higher latitudes will experience. But these same locations will have limited light, a trend that studies examining temperature alone have missed.

"The reason solar radiation is an interesting factor here is because most areas that will surpass the lower temperature tolerance for plant growth — meaning an area will become warm enough to support plants in the future but do not support them today — will still be limited by solar radiation," said Jamie Caldwell, referring to the effects of Earth's shape and rotation at these elevations.

"Regions at higher latitudes will likely have less frost and snow on the ground in the future, but many plants will not be able to take advantage of those warmer temperatures, because there will not be enough sunlight to sustain their growth," Iain Caldwell said.

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While this is not the first study to dispel the idea that plants will benefit from climate change — other, regional studies, for example, have shown such effects — the new paper takes a broader perspective, "examining what will happen to the majority of the world's plant growth given factors that limit that growth," Caldwell said.

Despite the potential for hardship under ongoing emissions rates, the study found that some northern regions — predominantly in China, Russia and Canada — likely will experience improved climatic conditions for plant growth.

"Our study provides important policy implications," Mora said. "It suggests that we should make better friends with the Canadians. Not to make light of the situation, but imagine the political leverage that climate change could give to some countries if they gain the power to feed the rest of the world."

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