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Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.

[1] In this study, we present the collation and analysis of the gridded land-based dataset of indices of temperature and precipitation extremes: HadEX2. Indices were calculated based on station data using a consistent approach recommended by the World Meteorological Organization (WMO) Expert Team on Climate Change Detection and Indices, resulting in the production of 17 temperature and 12 precipitation indices derived from daily maximum and minimum temperature and precipitation observations. High-quality in situ observations from over 7000 temperature and 11,000 precipitation meteorological stations across the globe were obtained to calculate the indices over the period of record available for each station. Monthly and annual indices were then interpolated onto a 3.75 Â 2.5 longitude-latitude grid over the period 1901-2010. Linear trends in the gridded fields were computed and tested for statistical significance. Overall there was very good agreement with the previous HadEX dataset during the overlapping data period. Results showed widespread significant changes in temperature extremes consistent with warming, especially for those indices derived from daily minimum temperature over the whole 110 years of record but with stronger trends in more recent decades. Seasonal results showed significant warming in all seasons but more so in the colder months. Precipitation indices also showed widespread and significant trends, but the changes were much more spatially heterogeneous compared with temperature changes. However, results indicated more areas with significant increasing trends in extreme precipitation amounts, intensity, and frequency than areas with decreasing trends.Citation: Donat, M. G., et al. (2013), Updated analyses of temperature and precipitation extreme indices since the beginning of the twentieth century: The HadEX2 dataset,

Article type: Letter 41 42 One Sentence Summary: Marine heatwaves alter ecosystem structure and functioning at 43 global scales. 44 45 46 47 48 49 implications for marine ecosystems 1 . Concurrent with long-term persistent warming, 50 discrete periods of extreme regional ocean warming (marine heatwaves, 'MHWs') have 51 increased in frequency 2 . Here we quantify trends and attributes of MHWs across all 52 ocean basins and examine their biological impacts from species to ecosystems. Multiple 53 regions within the Pacific, Atlantic and Indian Oceans are particularly vulnerable to 54 MHW intensification, due to the co-existence of high levels of biodiversity, a prevalence 55 of species found at their warm range edges, or concurrent non-climatic human impacts. 56 The physical attributes of prominent MHWs varied considerably, but all had 57 deleterious impacts across a range of biological processes and taxa, including critical 58 foundation species (corals, seagrasses and kelps). MHWs, which will likely intensify 59 with anthropogenic climate change 3 , are rapidly emerging as forceful agents of 60 disturbance with the capacity to restructure entire ecosystems and disrupt the provision 61 of ecological goods and services in coming decades. 62 63 Anthropogenic climate change is driving the redistribution of species and reorganization of 64 natural systems and represents a major threat to global biodiversity 4,5 . The biosphere has 65 401 working group on marine heatwaves (www.marineheatwaves.org).

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Global temperature targets, such as the widely accepted 2°C limit, may fail to 12 communicate the urgency of reducing CO 2 emissions. Translation of CO 2 emissions 13 into regional-and impact-related climate targets could be more powerful because 14 they resonate better with national interests. We illustrate this approach using 15 regional changes in extreme temperatures and precipitation. These scale robustly 16 with global temperature across scenarios, and thus with cumulative CO 2 emissions. 17 This is particularly relevant for changes in regional extreme temperatures on land, 18 which are much greater than changes in the associated global mean. 19 20 The IPCC 5 th Assessment Report included a figure in the Summary for Policymakers 21 (SPM) of the Working Group 1 (WG1) that linked global mean temperature changes 22 (ΔT glob) to total CO 2 emissions from 1870 onwards 1 (Fig. 1). This figure is compelling 23 because it shows a clear linear relationship between cumulative CO 2 emissions and a 24 measure of the global climate response. The obvious consequences are that every ton of 25 CO 2 contributes about the same amount of global-scale warming, no matter when it is 26 emitted, that any target for the stabilization of ΔT glob implies a finite CO 2 budget or quota 27 that can be emitted, and that global net emissions at some point need to be zero 2,3,4,5,6. 28 29 This simple relationship between CO 2 emissions and changes in ΔT glob (Fig. 1) has helped 30 overcome one communication barrier for the public in relating greenhouse gas emissions 31 with the climate system response. Yet, another obstacle remains the actual appreciation of 32 associated climate impacts, namely the translation of changes in global mean temperature 33 to regional-scale consequences for society and the environment. In this Perspective, we 34 demonstrate the feasibility of-as well as make the case for-quantitatively relating 35 global-scale cumulative CO 2 emissions to regional climate targets. We illustrate this 36 approach by scaling changes in hot and cold extreme temperatures and heavy 37 precipitation events with changes in the global mean temperature. 38 39 Global vs regional climate targets 40 Our experience shows that the implications of projected global mean temperature 41 changes tend to be underestimated at regional (and country) level, because these are 42 much smaller than the expected changes in regional temperature mean and extremes over 43 most land areas 7,8,9,10. The limitations of focusing on global mean temperature as a 44 measure of climate change has, for instance, been evidenced by the public debate about 45 the recent "hiatus". This has fixated attention on changes in ΔT glob instead of the 46 discernible worldwide impacts of the continued increases in radiative forcing 1 ,11,12,13,14. 47

Marine heatwaves (MHWs) can cause devastating impacts to marine life. Despite the serious consequences of MHWs, our understanding of their drivers is largely based on isolated case studies rather than any systematic unifying assessment. Here we provide the first global assessment under a consistent framework by combining a confidence assessment of the historical refereed literature from 1950 to February 2016, together with the analysis of MHWs determined from daily satellite sea surface temperatures from 1982–2016, to identify the important local processes, large-scale climate modes and teleconnections that are associated with MHWs regionally. Clear patterns emerge, including coherent relationships between enhanced or suppressed MHW occurrences with the dominant climate modes across most regions of the globe – an important exception being western boundary current regions where reports of MHW events are few and ocean-climate relationships are complex. These results provide a global baseline for future MHW process and prediction studies.

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