Climate Change in the Age of Humans

CURT STAGER

Human-driven climate change is only one of many challenges that forests must face during the twenty-first century and beyond. Even without adding more heat-trapping carbon dioxide to the atmosphere than would be available should all the planet's volcanoes erupt at once (Gerlach 2011), the presence of billions of human beings on Earth represents a major source of environmental change. We have become so numerous, our technologies so powerful, and our societies so interconnected that we have become a force of nature on a geological scale.

There is no consensus yet on when the Anthropocene epoch began (Crutzen and Stoermer 2000; Stager 2011). Most definitions date it to the Industrial Revolution, but human impacts on what were previously thought to be "untouched" landscapes have long affected forests through megaherbivore extinctions, land clearance, fires, grazing, and cultivation (Willis et al. 2004; Willis and Birks 2006; Lorenzen et al. 2011). Although its authorship and timing are difficult to pin down, the Anthropocene concept nevertheless provides a useful context for ecosystem management.

With approximately one-quarter of the planet's carbon dioxide reservoir now attributable to fossil fuel emissions, our behavior has become an integral part of global ecology. Our artificial nitrogen fixation now matches or exceeds natural production of available nitrogen worldwide; we change the appearances of continents through land use practices, rising sea levels, and shrinking ice masses; we disperse some species widely while driving others to extinction; and we guide evolution through changes in gene flow, selective breeding, and genetic engineering. The human presence affects the distribution, reproduction, and community structure of forests as well as their very survival, and it will make the ecological consequences of future climatic changes unique in the history of the planet.

Theoretical modeling provides possible examples of what may lie ahead in terms of climate, but proxy records of geologic history can also help to show which scenarios are most realistic and provide examples of biotic responses to climatic shifts in the past.

Today's anthropogenic climatic effects are superimposed on a background of variability that includes cyclic and irregular fluctuations on multiple spatial and temporal scales. Long, high-resolution records from ice cores, tree rings, cave formations, and aquatic sediments show that abrupt and extreme climate events are not limited to human causes, and that many of today's tree taxa have experienced such changes before.

The last 50 million years of the Cenozoic era was dominated by cooling from the high- CO2 hothouse of the Eocene "climatic optimum" (figure 1.1). The reasons for this are still unclear, but tectonism, weathering of the continents, and sequestration of carbon in marine sediments are likely contributors to the cooling trend (Garzione 2008). Temperatures fell low enough for an Antarctic ice cap to form between 45 and 34 million years ago, and during the last 3 million years temperatures have dropped far enough to trigger several dozen ice ages.

The overall cooling pattern of the Cenozoic was also punctuated by abrupt warming events. One of the most commonly cited examples was the PETM (Paleocene-Eocene Thermal Maximum) that occurred 56 million years ago and lasted roughly 200,000 years (figure 1.1; Dickens 2011). Atmospheric CO2 concentrations are thought to have reached or exceeded 3,000 ppm following the release of several thousand gigatons (GT; billions of metric tons) of carbon-rich gases into the atmosphere, possibly through volcanism in the Atlantic basin as well as other factors (Pearson and Palmer 2000; Dickens 2011). Global average temperatures rose 5°–10°C above their already warm states within 20,000 years or less, plant species migrated poleward, and insect herbivory on foliage increased, possibly in response to higher temperatures (Wing et al. 2005; Currano et al. 2008). Deciduous redwood forests encircled the Arctic Ocean, Nothofagus beech forests covered Antarctica, and ice-free, richly vegetated continents and land bridges facilitated the rapid migration of species (Bowen et al. 2002;Smith et al. 2006; Williams, Mendell, et al. 2008; Cantrill and Poole...