Fire Weather Compromises Large Scale Afforestation Scenarios

Ambitious climate change mitigation scenarios typically include substantial amounts of carbon dioxide removal. Such negative emissions are projected by integrated assessment models (IAMs) to be partly provided by afforestation and reforestation. At present, only few IAMs incorporate climate information or natural forest disturbances of any kind on forest dynamics. In this study we show how exposed to fire weather the afforestation areas in the IAM projections are. We illustrate that IAM mitigation scenarios lack climate information to arrive at more realistic projections of carbon stocks in forests, more reliable land use distributions and more realistic forestation costs. In this work we combine forest fractional cover from IAM land use projections and fire weather index (FWI) from multi-model climate projections, based on the latest simulations assessed in the 6th assessment report of the IPCC. With this metric we show how forests and afforestation areas are and will be affected by fire weather. We find a strong upward trend of forest mean fire weather under a 2 °C warming scenario (SSP1-2.6 for both land use and climate, roughly compatible with 2.6 W/m² climate forcing) driven by afforestation more than by fire weather intensification, increasing exposure by 27 % by the end of the century. We argue that climate information, especially climate forcing of forest disturbances (fires, hot droughts, etc.) needs to be included in the modelling framework of IAMs. Such developments would enhance the consistency between emission and climate projections. While such efforts are underway, IAM scenarios currently available likely underestimate management cost of large scale afforestation or overestimate the effectiveness and hence the remaining carbon budget for positive emissions.