Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state‐of‐the‐art FORest Canopy‐Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave–drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress‐induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model‐measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long‐term ecosystem‐scale observations to enable improved model representation of atmosphere–biosphere interactions in a changing global climate.

Isoprene concentrations were measured at four heights below, within, and above the forest canopy in Wytham Woods (United Kingdom) throughout the summer of 2018 using custom‐built gas chromatographs (the iDirac). These observations were complemented with selected ancillary variables, including air temperature, photosynthetically active radiation, occasional leaf gas exchange measurements, and satellite retrievals of normalized difference vegetation and water indices. The campaign overlapped with a long and uninterrupted heatwave accompanied by moderate drought. Peak isoprene concentrations during the heatwave‐drought were up to a factor of 4 higher than those before or after. Higher temperatures during the heatwave could not account for all the observed isoprene; the enhanced abundances correlated with drought stress. Leaf‐level emissions confirmed this and also included compounds associated with ecosystem stress. This work highlights that a more in‐depth understanding of the effects of drought stress is required to better characterize isoprene emissions.

Mitigation of greenhouse gas emissions is crucial for achieving the goals of the Paris climate agreement. One key gas is methane, whose representation in most climate models is limited by using prescribed surface concentrations. Here we use a new, methane emissions-driven version of the UK Earth System Model (UKESM1) and simulate a zero anthropogenic methane emissions scenario (ZAME) in order to (i) attribute the role of anthropogenic methane emissions on the Earth system and (ii) bracket the potential for theoretical maximum mitigation. We find profound, rapid and sustained impacts on atmospheric composition and climate, compared to a counterfactual projection (SSP3-7.0, the ’worst case’ scenario for methane). In ZAME, methane declines to below pre-industrial levels within 12 years and global surface ozone decreases to levels seen in the 1970s. By 2050, 690,000 premature deaths per year and 1° of warming can be attributed to anthropogenic methane in SSP3-7.0. This work demonstrates the significant maximum potential of methane emissions reductions, and their air-quality co-benefits, but also reiterates the need for action on carbon dioxide (CO2) emissions. We show that a methane emissions-driven treatment is essential for simulating the full Earth system impacts and feedbacks of methane emissions changes.