Seasonal mean temperature changes control future heat waves.

Increased temperature will result in longer, more frequent, and more intense heat waves. Changes in temperature variability have been deemed necessary to account for future heat wave characteristics. However, this has been quantified only in Europe and North America, while the rest of the globe remains unexplored. Using late century global climate projections, we show that annual mean temperature increases is the key factor defining heat wave changes in most regions. We find that commonly studied areas are an exception rather than the standard and the mean climate change signal generally outweighs any influence from variability changes. More importantly, differences in warming across seasons are responsible for most of the heat wave changes and their consideration relegates the contribution of variability to a marginal role. This reveals that accurately capturing mean seasonal changes is crucial to estimate future heat waves and reframes our interpretation of future temperature extremes.

Figure 3. Proportion of heat wave changes explained by temperature variability changes. (a, b) Proportion of changes in heat wave frequency, (c, d) mean duration of heat waves, and (e, f ) heat wave intensity explained by temperature variability changes using an annual (a, c, and e) and seasonal (b, d, and f ) mean shift. Stippling indicates areas where more than half of the models show a significant difference between scenarios with and without variability changes and at least 80% of them agree in the sign of the difference (see section 2). White masked areas indicate regions where the longest heat wave in all models spans a period of half a year or longer.