Coral reefs support immense biodiversity and provide important ecosystem services to many millions of people Yet reefs are degrading rapidly in response to numerous anthropogenic drivers In the coming centuries, reefs will run the gauntlet of climate change, and rising temperatures will transform them into new configurations, unlike anything observed previously by humans Returning reefs to past configurations is no longer an option Instead, the global challenge is to steer reefs through the Anthropocene era in a way that maintains their biological functions Successful navigation of this transition will require radical changes in the science, management and governance of coral reefs

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Coral reefs in the Anthropocene

The human impact on life on Earth has increased sharply since the 1970s, driven by the demands of a growing population with rising average per capita income. Nature is currently supplying more materials than ever before, but this has come at the high cost of unprecedented global declines in the extent and integrity of ecosystems, distinctness of local ecological communities, abundance and number of wild species, and the number of local domesticated varieties. Such changes reduce vital benefits that people receive from nature and threaten the quality of life of future generations. Both the benefits of an expanding economy and the costs of reducing nature's benefits are unequally distributed. The fabric of life on which we all depend-nature and its contributions to people-is unravelling rapidly. Despite the severity of the threats and lack of enough progress in tackling them to date, opportunities exist to change future trajectories through transformative action. Such action must begin immediately, however, and address the root economic, social, and technological causes of nature's deterioration.

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Pervasive human-driven decline of life on Earth points to the need for transformative change

Arctic feedbacks accelerate climate change through carbon releases from thawing permafrost and higher solar absorption from reductions in the surface albedo, following loss of sea ice and land snow. Here, we include dynamic emulators of complex physical models in the integrated assessment model PAGE-ICE to explore nonlinear transitions in the Arctic feedbacks and their subsequent impacts on the global climate and economy under the Paris Agreement scenarios. The permafrost feedback is increasingly positive in warmer climates, while the albedo feedback weakens as the ice and snow melt. Combined, these two factors lead to significant increases in the mean discounted economic effect of climate change: +4.0% ($24.8 trillion) under the 1.5 °C scenario, +5.5% ($33.8 trillion) under the 2 °C scenario, and +4.8% ($66.9 trillion) under mitigation levels consistent with the current national pledges. Considering the nonlinear Arctic feedbacks makes the 1.5 °C target marginally more economically attractive than the 2 °C target, although both are statistically equivalent. Nonlinear transitions in permafrost carbon feedback and surface albedo feedback have largely been excluded from climate policy studies. Here the authors modelled the dynamics of the two nonlinear feedbacks and the associated uncertainty, and found an important contribution to warming which leads to additional economic losses from climate change.

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Climate policy implications of nonlinear decline of Arctic land permafrost and other cryosphere elements

The 2015 Paris Agreement calls for countries to pursue efforts to limit global-mean temperature rise to 1.5 °C. The transition pathways that can meet such a target have not, however, been extensively explored. Here we describe scenarios that limit end-of-century radiative forcing to 1.9 W m−2, and consequently restrict median warming in the year 2100 to below 1.5 °C. We use six integrated assessment models and a simple climate model, under different socio-economic, technological and resource assumptions from five Shared Socio-economic Pathways (SSPs). Some, but not all, SSPs are amenable to pathways to 1.5 °C. Successful 1.9 W m−2 scenarios are characterized by a rapid shift away from traditional fossil-fuel use towards large-scale low-carbon energy supplies, reduced energy use, and carbon-dioxide removal. However, 1.9 W m−2 scenarios could not be achieved in several models under SSPs with strong inequalities, high baseline fossil-fuel use, or scattered short-term climate policy. Further research can help policy-makers to understand the real-world implications of these scenarios.

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Scenarios towards limiting global mean temperature increase below 1.5 °C

Recent accelerated climate change has exacerbated existing environmental problems in the Mediterranean Basin that are caused by the combination of changes in land use, increasing pollution and declining biodiversity. For five broad and interconnected impact domains (water, ecosystems, food, health and security), current change and future scenarios consistently point to significant and increasing risks during the coming decades. Policies for the sustainable development of Mediterranean countries need to mitigate these risks and consider adaptation options, but currently lack adequate information — particularly for the most vulnerable southern Mediterranean societies, where fewer systematic observations schemes and impact models are based. A dedicated effort to synthesize existing scientific knowledge across disciplines is underway and aims to provide a better understanding of the combined risks posed.

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Climate change and interconnected risks to sustainable development in the Mediterranean

. Robust appraisals of climate impacts at different levels of global-mean temperature increase are vital to guide assessments of dangerous anthropogenic interference with the climate system. The 2015 Paris Agreement includes a two-headed temperature goal: "holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C". Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing. Here we provide an assessment of key impacts of climate change at warming levels of 1.5 °C and 2 °C, including extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss. Our results reveal substantial differences in impacts between a 1.5 °C and 2 °C warming that are highly relevant for the assessment of dangerous anthropogenic interference with the climate system. For heat-related extremes, the additional 0.5 °C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions. Similarly, this warming difference is likely to be decisive for the future of tropical coral reefs. In a scenario with an end-of-century warming of 2 °C, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards. This fraction is reduced to about 90 % in 2050 and projected to decline to 70 % by 2100 for a 1.5 °C scenario. Analyses of precipitation-related impacts reveal distinct regional differences and hot-spots of change emerge. Regional reduction in median water availability for the Mediterranean is found to nearly double from 9 % to 17 % between 1.5 °C and 2 °C, and the projected lengthening of regional dry spells increases from 7 to 11 %. Projections for agricultural yields differ between crop types as well as world regions. While some (in particular high-latitude) regions may benefit, tropical regions like West Africa, South-East Asia, as well as Central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize. Best estimate sea-level rise projections based on two illustrative scenarios indicate a 50 cm rise by 2100 relative to year 2000-levels for a 2 °C scenario, and about 10 cm lower levels for a 1.5 °C scenario. In a 1.5 °C scenario, the rate of sea-level rise in 2100 would be reduced by about 30 % compared to a 2 °C scenario. Our findings highlight the importance of regional differentiation to assess both future climate risks and different vulnerabilities to incremental increases in global-mean temperature. The article provides a consistent and comprehensive assessment of existing projections and a good basis for future work on refining our understanding of the difference between impacts at 1.5 °C and 2 °C warming.

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Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 °C and 2 °C

Comprehensive computer simulations show that coral reefs are likely to suffer extensive long-term degradation resulting from mass bleaching events even if the expected increase in global mean temperature can be kept well below 2 °C Without major mitigation efforts to limit global warming significantly, the fate of coral reef ecosystems seems to be sealed

Limiting global warming to 2 °C is unlikely to save most coral reefs

Mountain channels can be strongly coupled with adjacent hillslopes, exchanging both mass and energy. However, hypotheses of the underlying cause and effect relations are based on indirect observations that do not resolve the mechanics of channel-hillslope coupling at the process scale. Here we present direct observational data of a coupled channel-hillslope system in the catchment area of the Erlenbach, a mountain stream in Switzerland. A slow-moving landslide flanking this alpine stream failed after a flood had eroded an alluvial step in the channel at its base, representing evidence for an upsystem link in channel-hillslope coupling. Progressive accumulation of landslide debris in the channel resulted in a renewed step, stabilizing the hillslope and restoring the channel step in a downsystem link. Thus, upsystem and downsystem coupling mechanisms are joined in a negative feedback cycle. In this cycle, debuttressing and rebuttressing due to channel bed erosion and alluviation are the dominant controls on hillslope stability. Based on an order of magnitude estimate it is plausible that the observed feedback mechanism is a relevant process in the production of coarse (>2 mm) sediment in the Erlenbach.

Controls and feedbacks in the coupling of mountain channels and hillslopes

Testing models of step formation against observations of channel steps in a steep mountain stream

 Landscape patterns result from landscape forming processes This link can be exploited in geomorphological research by reversely analyzing the geometrical content of landscapes to develop or confirm theories of the underlying processes Since rivers represent a dominant control on landscape formation, there is a particular interest in examining channel metrics in a quantitative and objective manner For example, river cross-section geometry is required to model local flow hydraulics, which in turn determine erosion and thus channel dynamics Similarly, channel geometry is crucial for engineering purposes, water resource management, and ecological restoration efforts These applications require a framework to capture and derive the data In this paper we present an open-source software tool that performs the calculation of several channel metrics (length, slope, width, bank retreat, knickpoints, etc) in an objective and reproducible way based on principal bank geometry that can be measured in the field or in a GIS Furthermore, the software provides a framework to integrate spatial features, for example the abundance of species or the occurrence of knickpoints The program is available at https://githubcom/AntoniusGolly/cmgo and is free to use, modify, and redistribute under the terms of the GNU General Public License version 3 as published by the Free Software Foundation

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Antonius Golly

Papers

Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 °C and 2 °C

Limiting global warming to 2 °C is unlikely to save most coral reefs

Controls and feedbacks in the coupling of mountain channels and hillslopes

Testing models of step formation against observations of channel steps in a steep mountain stream

Deriving principal channel metrics from bank and long-profile geometry with the R package cmgo