Climate Change

The Earth’s climate has always been changing. There have been four ice ages in the past 450,000 years. Surprisingly, analyses of mile-deep ice cores taken from the Greenland ice sheet in the late 1900s has shown that during the last ice age between 18,000 to 80,000 years ago there were 25 abrupt changes in climate with changes in temperature of 8-15C occurring in just a few decades1. Forest vegetation and associated biota have adapted and evolved in response to these changes over the past several hundred million years2 and these adaptive changes are still occurring. 

Current, rapid changes in the Earth’s climate due to the emission of carbon dioxide from burning fossil fuels is evident through measures of atmospheric and ocean temperatures, sea-level rise, and increased frequency and intensity of storms and wildfires. In the 1960s the atmospheric CO2 level was 317 parts per million (ppm); in 2022 it is 417 ppm. What will it be in the future?

 It is anticipated that the Earth’s temperature increase must be kept below 1.5C, which is regarded as an irreversible tipping point beyond which recovery might not be possible. As documented by the International Panel on Climate Change, 2021,3 forestry can make a very significant contribution to low-cost global mitigation, particularly by avoiding converting forests to other land uses. The role of forests in the carbon cycle is shown in Fig. 1 in which trees take up CO2 and emit O2 as a ‘waste product’ which we need to breathe – what a system!

Tree and Stand Growth

In evaluating the capacity of forests to take up and store carbon it is important to consider the effects of tree age, health, and vigor.  Younger, vigorous trees are more efficient in taking up carbon because the rate of carbon dioxide taken up through photosynthesis exceeds the loss of carbon through respiration. Older trees and forests have higher storage, but as they become older the rate of net carbon uptake becomes lower because the amount of carbon lost through respiration and decomposition becomes larger. Managed forests with a mix of younger and older stands tend to sequester more carbon in the long run, particularly when including the carbon stored in forest products and the substitution of wood for alternative products that have higher energy production requirements. Additionally, managed forests have lower risk of loss of carbon through wildfires.

Effects of Climate Change

It is difficult to accurately predict the effects of climate change in California due to the diverse topography and diverse vegetation types (See Topic 1).  It seems likely, however, that the climate will be warmer and drier with precipitation changing from predominantly snow to mostly rain. Some areas on the coast could become wetter depending on changes in coastal fog. These generally drier conditions and more prevalent storms will likely increase the incidence and severity of large wildfires.

Already forest vegetation has been observed to be moving north in latitude (chaparral and oak woodland) and upward in elevation (ponderosa pine to oak woodland,  mixed conifer to ponderosa pine). Particularly at risk are those forest species with small ranges such as Joshua tree, St. Lucia fir, bristlecone pine, and Torrey pine. However, given the complexity of topography, these species may continue to exist in small, isolated areas. As well as forest vegetation moving, novel ecosystems could develop due to differences in adaptation rates of animals, birds, insects/disease, and pollinators.

Forest Management Considerations

In past regeneration practices, to ensure tree seedlings were adapted to the area being reforested, seed sources were selected from zones of equivalent elevation and latitude from among California’s 83 seed zones. Given that climate is becoming warmer and drier, managers are considering using seed from zones that are lower in elevation and from southern slopes that may be better adapted to predicted changes in climate (See Topic 3).

Due to uncertainties of climate change it is important to manage forests for diversity and to keep them vigorous and resilient to impacts such as pest epidemics and wildfire. The most effective way to do this is through controlling stand density (See Topics 5 and 6).

Carbon Credits

Forestland owners can gain carbon credits through plans approved by the California Air Resources Board by managing their lands through carbon offset and cap-and-trade programs. Qualifying projects must demonstrate that carbon sequestration is ‘permanent’ and greater than ‘business as usual’4. This often involves growing trees for longer rotations and to larger sizes. Owners of small, family forests are also able to gain access to carbon markets through programs such as those administered by the American Forest Foundation5.


The effects of climate change can be mitigated through keeping forests as forests and managing them to keep them vigorous and healthy. Replacing fossil fuels with renewable sources of energy, including woody biomass, remains the highest priority. And using wood for furniture, buildings and other products not only uses a renewable natural resource, but uses less energy in manufacture thus putting less CO2 into the atmosphere.