We’ve heard the predictions of how greenhouse gas emissions will drive changes in the temperatures and precipitation people experience. But how these changes affect the world’s forests has broad implications for the future as well.

Could warmer winters, and thus longer growing seasons, cause trees to grow faster? If so, perhaps faster tree growth could slow the pace of climate change, since trees suck carbon out of the air as they grow.

Or perhaps hotter summers will mean more drought-like conditions, thereby hampering trees’ ability to grow and thus cause deterioration of our woodlands.

In a recent paper, my colleagues and I set out to make a map of how climate change might influence tree growth across the entire continent of North America. To do this, we dug into historical records of tree growth over the period 1900-1950 collected by many dedicated field ecologists over the decades and deposited in the International Tree Ring Data Bank.

What we found was that the daily life of trees across much of North America will become more challenging, despite the potential benefit that rising carbon dioxide concentrations may have for trees. This is contrary to some scientists’ hopes that climate change will strongly benefit northern latitude forests.

How trees respond to climate

The first hurdle in predicting future tree growth is to understand how trees in different ecosystems respond to climate fluctuations.

You might guess that in cold northern forests, a little heat might help trees grow, whereas more heat in the desert Southwest is likely the last thing trees there want. This observation motivated previous scientists to formulate a “boreal greening” hypothesis – that global warming will cause northern boreal forests to grow faster and help mitigate climate change.

We used the historic tree ring data to map the relationship between regional climate and tree growth. Matching each growth ring to the weather patterns in the corresponding year, we can get a sense for how trees respond to climate fluctuations. For instance, we saw that above-average June temperatures caused faster tree growth in places with climates similar to Fairbanks, Alaska, but slower growth in Phoenix-like climates.

As the climate changes, we might expect the response of trees to change as well. For example, in Fairbanks, our models actually predict that, in the future, above-average June temperatures will be bad for tree growth there, which is opposite of the historic relationship. Why? Fairbanks warms up so much that it shifts to a new climatic zone in which additional warming is now a detriment. Other researchers have actually started to see such a shift occur on the ground in Alaska.

Once we characterize how trees respond to changes in climate across the continent, we can use the forecasts from the U.N.‘s Intergovernmental Panel on Climate Change (IPCC) to predict the corresponding change in tree growth across the continent. For each pixel on our map of North America, we projected how forests will change based on both sets of information – the growth-climate relationship we established through the tree ring analysis and the projected changes in climate in the continent.

Carbon fertilization

There is one more wrinkle to this puzzle that we examined. The changing climate is driven largely by a buildup of additional carbon dioxide, and plants use carbon dioxide to photosynthesize. Just as we breathe in oxygen to live, plants breathe in carbon dioxide to live. Thus, increased amounts of carbon dioxide might directly speed up tree growth. This is known as “carbon fertilization” because it’s like we are adding fertilizer to the plants through the air to help them grow.

Scientists are deeply divided about whether carbon fertilization of this type will actually cause increases in growth, and if so, how much. In our paper, we did not attempt to settle this debate. Instead, we just included multiple different possibilities for the strength of carbon fertilization.

To simulate carbon fertilization, we used a neat little trick suggested by Professor Graham Farquhar of Australian National University. The trick relies on the fact that as plants breathe in carbon dioxide, water escapes. Think of the pores on leaves as little mouths that open and close to breathe. The more plants need to open their mouths to breathe, the more water escapes. So plants try to keep their mouths as tightly closed as they can.

If the concentrations of carbon dioxide floating around in the air are very high, plants need open their mouths only a little bit for a small gulp of air without losing much water. Thus, as we fertilize the plants with carbon in the air, this directly increases the amount of water the plants are able to retain – with more CO2, the leaves’ pores will absorb the gas more efficiently and in the process lose less water.

Instead of trying to simulate more free carbon floating around in the air, we can just pretend that the plants receive more rainwater. The ultimate effect on growth should be essentially the same, because carbon uptake and water retention are directly linked.

In deserts where water is at a premium and plants are highly motivated to keep their mouths shut, a little carbon fertilization (or a little extra rain) should go a long way toward helping plants grow. By contrast, in rainforests where plants can keep their mouths wide open with little cost, carbon fertilization (or extra rain) might not do much to help the plants.

In our study, we simulated carbon fertilization by simply adding more future precipitation into our models. To satisfy those scientists who strongly believe that carbon fertilization will pan out, in some simulations we added extra water in proportion to the amount of extra carbon that is projected to be released into the atmosphere. To satisfy the nay-saying scientists who don’t believe the carbon fertilization effect will pan out, we also ran simulations without any increased water. And we ran simulations at all levels in between.

Our models’ predictions

At the end of the day, our maps of how tree growth might respond to climate change are alarming.

Across much of the west and central parts of the continent, we see massive decreases in tree growth rates, with trees growing up to 75 percent slower by the second half of this century. However, in some areas near the continent’s coasts, such as the Pacific Northwest, western Canada and the southeastern United States, we saw some local increases in tree growth rates.

On average, without the carbon fertilization effect, our models project growth rates across the continent to fall by almost 20 percent under the worst-case climate change scenario put forth by the IPCC (this scenario has 6 degrees Celsius of warming forecast across the continent).

We found that it would take a very large carbon fertilization effect (unrealistically large, according to the opinion of several of our study’s co-authors) to offset this slowdown. And across much of the continent, our models projected slower growth rates no matter how large the carbon fertilization effect.

Also, we did not see a large increase in cold northern forest growth rates in our simulations. So, on average, we saw no “boreal greening.” If anything, we saw a slowdown of these forests. This is largely driven by the shift in how trees respond to climates in places like Fairbanks.

What it means

The implication of our analysis is that forests do not seem poised to save us from climate change.

Our models suggest that most of our forests will be growing more slowly in the future. This will, of course, have direct impacts on all the ways we and other species rely on trees. But it will also feed back into climate change itself. As global warming causes trees to absorb less carbon, there will be more carbon left in the air to cause faster warming, thus creating an accelerating cycle.

Furthermore, many sustained years of bad growth in trees will likely deplete the resources they need to survive, making them susceptible to severe droughts or insect outbreaks. This may mean that what we project as slower growth may translate into widespread tree death. In other words, the forest picture may be even gloomier than our models suggest.

In our models, we don’t take into account the way forests are changing due to changes in logging practices or forest management. In many areas, forests are regrowing faster simply because we stopped logging them recently. Such factors should be thought of as another layer to add on top of our projections.

This study, like any of its kind, is really our best guess at approximating the future. I think of such forecasts not as hard-and-fast predictions of what will happen, but as reasonable possibilities. There are so many unknowns involved, including the fact that future climates will likely be quite different from any we have seen in the past.

And of course the biggest unknown is how much willpower our human community will bring to the cause of clamping down on greenhouse gas emissions.

This article was updated on July 27, 2016 to say higher levels of carbon dioxide in air result in an increase, rather than decrease, in the amount of water plants can retain.