“We have nothing comparable in the past to today’s environment and certainly tomorrow’s environment. With increasing drought stress, our forests of tomorrow will hardly resemble our forests of yesterday.”
—Henri Grissino-Mayer, geography professor at the University of Tennessee, Knoxville

When you want to look at evidence of how dry and how wet conditions have been in the last 1,000 years, one good way to do it is to look at tree rings. That’s because tree rings grow more slowly in drier conditions; during periods of relative drought the rings will be narrow. During wetter seasons, the rings will be more widely spaced apart. It’s a great way for paleoclimatologists to look back into the past and make determinations of the conditions local regions were subject to, a subject of study known as dendrochronology.

Henri Grissino-Mayer is without a doubt a dendrochronologist. As reported in Science Daily earlier this week, he has been working with scientists from the Los Alamos National Laboratory led by Park Williams, the U.S. Geological Survey, the University of Arizona and Columbia University. Together they have looked back over the last millennium specifically at coniferous trees—evergreens with needles and cones— in the American southwest.

During that time, there have indeed been mega-droughts. However, the researchers have noted that the current one that part of the US has been experiencing which started in the late 1990s is looking to become one of the worst, if not the very worst if present trends continue. Appreciating that there are other factors that can influence the health of forests such as bark-beetle outbreaks and wildfires, the researchers developed a tree-ring-based index that looks at those stressors to trees as well as those related to moisture and drought.

Looking forward to the year 2050 with the models they have devised, Grissino-Mayer and his team predict worse drought and increased tree mortality than has been witnessed thus far.

As Grissino-Mayer puts it: “This [current] drought will be exacerbated by increasing temperatures globally, foreshadowing major changes in the structure and species composition of forests worldwide.”

The problems related to global warming will likely have a bigger impact on drier regions such as the American southwest than in other areas of North America. With global warming and climate change, an imbalance of water develops. Although warmer air will also become wetter air due to increased evaporation and a greater capacity to hold onto moisture, that evaporation doesn’t only come from bodies of water. It comes from soils as well. In fact, the evaporation from soils can be at an even quicker rate. Drier soil is bad for trees no matter how much moisture is int he air.

Grissino-Mayer predicts this is a problem that we will have to adapt to because it isn’t going to go away anytime soon. Forestry management practices in the driest regions of the US will need to be altered to maintain healthy forests, and preparation for increased wildfires will also be important.

The original paper was published online on September 30, 2012 in Nature Climate Change.