Does a Warmer World Mean a Greener World? Not Likely!

Despite the “gloom and doom” scenarios depicted by most climate change scientists, the warmer world that we are creating can’t be all bad, can it? After all, hundreds of studies have shown that plant productivity is higher when temperatures are warmer and atmospheric carbon dioxide is high. Many models even predict an increase in global plant productivity, which has provided fodder for many to advocate the benefits of climate change to humans. Why wouldn’t we want a warmer and CO2-enriched world if it means higher productivity of plants, especially in impoverished regions where even slight increases in plant production could mean the difference between starvation and prosperity? Unfortunately, the simple idea that global warming could provide at least some benefits to humanity by increasing plant production is complicated by a number of factors. It is true that fertilizing plants with CO2 and giving them warmer temperatures increases growth under some conditions, but there are trade-offs. While global warming can increase plant growth in areas that are near the lower limits of temperature (e.g., large swaths of Canada and Russia), it can make it too hot for plant growth in areas that are near their upper limits (e.g., the tropics). In addition, plant productivity is determined by many things (e.g., sunlight, temperature, nutrients, and precipitation), several of which are influenced by climate change and interact with one another. And so while we have hundreds of models that are available to project future climate change based on a number of different scenarios, we really have little clue as to what this future climate-changed world might look like in terms of plant production—that is, until now. In this issue of PLOS Biology, Mora and colleagues provide a broader view of how global warming will likely change plant productivity in several ways. First, they include multiple factors that influence plant growth and are expected to be affected by climate change—temperature, soil water content, and sunlight levels –and their interactions. Second, they examine expected changes in different regions across the globe, examining the net increases and decreases in the number of days suitable for plant growth under climate change. Third, they examine the correlations between these expected changes in different regions and the vulnerability of people living in those regions. The answers they find, in a nutshell, are that if carbon emissions remain on their current trajectory (the status quo), the losses of plant production are likely to be far greater than any gains when examined at a global scale, and a substantial number of people, especially those who are most impoverished, will be at greater risk. Importantly, however, Mora and colleagues repeated their analysis for several global change scenarios and found that these changes will be much more moderate if society agrees to restrict carbon emissions in real, but manageable, ways. A look into the nitty-gritty of Mora and colleagues’ analyses shines some light on how they arrived at these conclusions. First, they used remote sensing satellite data to correlate how plant growth responded to varying levels of temperature, soil moisture, and solar radiation.