Today the IPCC released its Special Report on the implications of a global temperature increase of 1.5°C above pre-industrial levels, and whether it is physically possible & feasible to achieve this climate stabilisation. The Summary for Policymakers: report.ipcc.ch/sr15/pdf/sr15_…
There are some key messages in the report. Bottom line: there is a substantial reduction in risks when stabilising at 1.5°C compared to 2°C; it is physically possible to achieve this lower stabilisation, but extremely challenging in infrastructure terms.
Human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels, with a likely range of 0.8°C to 1.2°C. Global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate.
Warming from past anthropogenic emissions will persist for centuries to millennia & will continue to cause further long-term changes in climate, such as sea level rise, with associated impacts, but these emissions alone are unlikely to cause global warming of 1.5°C.
Climate-related risks for natural & human systems are higher for warming of 1.5°C than now, but lower than at 2°C. These risks depend on magnitude & rate of warming, geographic location, levels of development & vulnerability, & on implementation of adaptation & mitigation options
Climate models project robust differences between 1.5°C and 2°C. These differences include increases in: mean temperature in most land & ocean regions, hot extremes in most inhabited regions, heavy precipitation in several regions & the probability of drought in some regions
By 2100, global mean sea level rise is projected to be around 0.1 metre lower with global warming of 1.5°C compared to 2°C. Sea level will continue to rise well beyond 2100, and the magnitude and rate of this rise depends on future emission pathways.
A slower rate of sea level rise enables greater opportunities for adaptation in the human and ecological systems of small islands, low-lying coastal areas and deltas.
On land, impacts on biodiversity & ecosystems, including species loss & extinction, are projected to be lower at 1.5°C of global warming compared to 2°C. Limiting global warming is projected to lower the impacts on ecosystems and to retain more of their services to humans.
Limiting global warming is projected to reduce increases in ocean temperature as well as associated increases in ocean acidity and decreases in ocean oxygen levels, & reduce risks to marine biodiversity, fisheries, and ecosystems, and their functions and services to humans.
Climate-related risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with global warming of 1.5°C and increase further with 2°C. 
Most adaptation needs will be lower for global warming of 1.5°C compared to 2°C. There are a wide range of adaptation options that can reduce risks of climate change. There are limits to adaptation & adaptive capacity for some human & natural systems at global warming of 1.5°C.
That's the summary of the physical climate science and implications. It offers policymakers a choice between different possible future worlds: 1.5°C, 2°C, or higher. Whether this is achievable or not, and with what costs, is where it gets more complicated....
In model pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO2 emissions decline by ~45% from 2010 levels by 2030, reaching net zero around 2050. For limiting to below 2°C CO2 emissions are projected to decline by ~20% by 2030 & reach net zero around 2075.
Non-CO2 emissions in pathways that limit global warming to 1.5°C show deep reductions that are similar to those in pathways limiting warming to 2°C.
Pathways limiting global warming to 1.5°C with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems.
These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors, a wide portfolio of mitigation options and a significant upscaling of investments in those options.
All pathways that limit global warming to 1.5°C project the use of CO2 removal (CDR) on the order of 100–1000 GtCO2 over the 21st century. CDR would be used to compensate for residual emissions and achieve net negative emissions to return global warming to 1.5°C following a peak.
CDR deployment of several hundreds of GtCO2 is subject to multiple feasibility and sustainability constraints. Significant near-term emissions reductions & measures to lower energy & land demand can limit CDR deployment to a few hundred GtCO2 without reliance on BECCS.
Estimates of the global emissions outcome of current nationally stated mitigation ambitions as submitted under the Paris Agreement would lead to global greenhouse gas emissions in 2030 of 52–58 GtCO2eq per year.
Pathways reflecting these ambitions would not limit global warming to 1.5°C, even if supplemented by very challenging increases in the scale & ambition of emissions reductions after 2030.
Avoiding overshoot & reliance on future large-scale deployment of carbon dioxide removal (CDR) can only be achieved if global CO2 emissions start to decline well before 2030.
The avoided climate change impacts on sustainable development, eradication of poverty & reducing inequalities would be greater if global warming were limited to 1.5°C rather than 2°C, if mitigation and adaptation synergies are maximized while trade-offs are minimized.
Adaptation options specific to national contexts, if carefully selected together with enabling conditions, will have benefits for sustainable development and poverty reduction with global warming of 1.5°C, although trade-offs are possible.
Mitigation options consistent with 1.5°C pathways are associated with multiple synergies & trade-offs across the Sustainable Development Goals (SDGs).
While the total number of possible synergies exceeds the number of trade-offs, their net effect will depend on the pace and magnitude of changes, the composition of the mitigation portfolio and the management of the transition.
Limiting risks from warming of 1.5°C in context of sustainable development & poverty eradication implies system transitions that can be enabled by increase of adaptation & mitigation investments, policy instruments, the acceleration of technological innovation & behaviour changes
Sustainable development supports, and often enables, the fundamental societal and systems transitions and transformations that help limit global warming to 1.5°C.
Such changes facilitate the pursuit of climate-resilient development pathways that achieve ambitious mitigation and adaptation in conjunction with poverty eradication and efforts to reduce inequalities.
Strengthening the capacities for climate action of national & sub-national authorities, civil society, the private sector, indigenous peoples and local communities can support the implementation of ambitious actions implied by limiting global warming to 1.5°C.
International cooperation can provide an enabling environment for this to be achieved in all countries and for all people, in the context of sustainable development. International cooperation is a critical enabler for developing countries and vulnerable regions. /fin
