Human-caused CO 2 emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO 2. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO 2. Water-use efficiency, namely the ratio of CO 2 uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO 2. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO 2 and contemporary climate change. Rising Atmospheric CO 2 and Global Climate Change Emissions of CO 2 associated with human industrial activity and land-use change over the past century have significantly impacted on global climate, causing global warming of about 1.0°C [1]. The anthropogenic CO 2 emission rate is continuing to increase, and the future rise in atmospheric CO 2 will undoubtedly lead to more climate change, including increases in the frequency of extreme climate events such as heatwaves, droughts, and storms [2]. Global climate change has the potential to significantly stress terrestrial vegetation [3], for example with hot, dry air, soil moisture deficits, or flooding. This could lead to a carbon-climate feedback in which widespread tree mortality and forest decline contribute to accelerating accumulation of CO 2 in the atmosphere [4-6]. On the other hand, plants interact directly with atmospheric CO 2 , and they can potentially respond to rising atmospheric CO 2 concentrations by increasing photosynthetic rates and water-use efficiency (see Glossary) [7-10]. Water-use efficiency in this context is defined as the amount of CO 2 taken up by photosynthesis for a given amount of water lost to the atmosphere by transpiration (Box 1). Understanding emergent responses of the production of terrestrial vegetation to the potentially opposing impacts of global climate change and CO 2 fertilization is crucial for formulating effective mitigation and adaptation strategies [11]. At a global scale, there is currently an imbalance between the amount of CO 2 absorbed by the terrestrial biosphere through photosynthesis and the amount released back to the atmosphere through plant respiration, decomposition, fire, and emissions from land-use change [12]. This is commonly referred to as the land carbon sink. It is slowing the rate of increase in atmospheric CO 2 that would otherwise result from anthropogenic CO 2 emissions. Predicting the future behaviour of the land carbon sink is one of the most important challenges in carbon cycle science, given the potential that feedbacks could accelerate the rate of future climate change [13]. This requires a thorough understanding of the process through which the terrestrial biosphere captures CO 2-photosynthesis. Highlights Global climate change caused by CO 2 emissions can stress terrestrial vegetation , potentially decreasing production. On the other hand, CO 2 interacts directly with plants, stimulating leaf-level photo-synthesis and water-use efficiency. The rise in atmospheric CO 2 concentration over the past century presents an opportunity for gauging the strength of the terrestrial biosphere response to these potential impacts. Atmospheric proxy and model analysis both suggest that global terrestrial photosynthesis has increased in nearly constant proportion to the rise in atmospheric CO 2 concentration, a maximal response by the terrestrial biosphere. An accurate understanding of the impacts of climate change on terrestrial vegetation is essential for managing risks associated with human-caused climate change: gauging the historic response of terrestrial photosynthesis is an important step in this direction.