Politicized scientists have the nasty habit of making public claims that are only true for special situations without noting the special situations, or burying them below the fold.

Growing more forests in United States could contribute to global warming

Planting trees across the United States and Europe to absorb some of the carbon dioxide emitted by the burning of fossil fuels may just outweigh the positive effects of sequestering that CO².

So, activist scientist Ken Caldeira concludes:

. . . in terms of climate change, we should focus our efforts on things that can really make a difference, like improving efficiency and developing new sources of clean energy.

But what does the study really show?

Using climate models, researchers from the Lawrence Livermore National Laboratory and the Carnegie Institution Department of Global Ecology have found that forests in the mid-latitude regions of the Earth, present a more complicated picture. Trees in these areas tend to warm the Earth in the long run. The darkness of these forests absorbs abundant sunlight, warming the land. While the darkness of the forest lasts forever, the effect of the forest sequestering carbon dioxide slows down over time as the atmosphere exchanges CO² with the ocean.

The conclusion: Planting a forest in the United States could cool the Earth for a few decades, but would lead to planetary warming in the long term. These are the results of a study that will be presented at 8:30 a.m. Wednesday, Dec. 7, at the American Geophysical Union Fall Meeting in San Francisco.

"On time scales longer than a few centuries, the net effect will actually be warming in these regions," said Govindasamy Bala of the Livermore team. "We thought planting trees across the northern hemisphere would help curb global warming by the CO² absorption but what we found was a different story."

Gee, we have to engage in empty gestures to make trivial reductions in emissions now because forests could cause warming in a few decades or centuries. It seems to me that a far smarter take on the issue would be that forests are good for now but that they aren't a long term solution if we don't develop cleaner energy technologies over the decades and centuries to come.

We will develop them of course so the warming scenario is not even remotely realistic. A far more sensible expectation is that planting forests now will help ease the consequences of past emissions and be a joy far into the future when we have developed better energy technologies. The are no realistic climate reasons to talk against forest planting.

There's a limit and a balance worth considering. As mentioned in many other posts grasslands planted to deep rooted perennials sequester lots of carbon. They also have a higher albedo than forests and reflect more sunlight back toward space. Converting cropland back to grassland would likely be more beneficial to climate than planting forests.

This isn't a decision we can make without regard to history and local conditions. Land that was cleared of forest for cropland probably still ought to be replanted to forest rather than grassland even though grass would be better from a climate perspective. We need both forests and grasslands.

Update:

How much do forests help?

Forest productivity may be significantly greater in an atmosphere enriched with carbon dioxide, according to findings released today that challenge recent reports that question the importance of carbon dioxide fertilization.

The study, performed by researchers at the Department of Energy's Oak Ridge National Laboratory and 10 other institutions in the United States and Europe, revealed a strong relationship between productivity of forest plots in the current atmosphere and productivity in plots experimentally enriched with carbon dioxide.

"The median response indicated a 23 percent increase in productivity in the future atmosphere," said ORNL's Rich Norby, lead author of the paper to be published Dec. 13 in the Proceedings of the National Academy of Sciences. "What was especially surprising to the research team was the consistency of the response across a wide range of productivity."

Researchers analyzed data from four experiments in which young forest stands were exposed for multiple years to an atmosphere with a carbon dioxide concentration predicted to occur in the middle of this century. The experiments were conducted in a deciduous forest in Tennessee, a pine forest in North Carolina, a young hardwood stand in Wisconsin and a high-productivity poplar plantation in Italy.

The team calculated net primary productivity – the annual fixation of carbon by green plants into organic matter – for each of the sites from data on wood, leaf and fine-root production. The results proved surprising. . .

"Although carbon dioxide fertilization of forests might slow the rate of increase of atmospheric carbon dioxide, a 23 percent increase in productivity is insufficient to stabilize the concentration in the atmosphere," he said. "The increase in productivity demonstrated in these experiments will most likely be tempered by the stresses of climate warming, ozone pollution or insufficient nitrogen supply. In addition, some of the increased organic matter entering the forest is not sequestered in wood but is rapidly returned to the atmosphere. Understanding the controls on carbon processing by ecosystems remains a priority research challenge."

What we really need to know is the amount of wood and root. Leaves are ephemeral.