Here’s the new IPCC report, Global Warming of 1.5 °C. I guess the idea is to show that this amount of warming, which most nations of the world have tentatively agreed to target, is still pretty bad. And the world is not even remotely on the path toward limiting warming to this level.

The report estimates the world has already warmed by about 1.0 degree C on average due to emissions that have already happened. If we stopped emissions today, the world would continue to warm, but warming would peak somewhere between 1.0 and 1.5 degrees C. I think this is an important concept to grasp – the effects that are beginning to be felt now are not the result of emissions happening now, but of past emissions including emissions decades ago. They would continue and get worse even if we stopped emitting today, and not only are we not lowering emissions, we are continuing and even accelerating them. So the problem is potentially one of runaway, exponentially growing consequences and we are only at the very beginning of the curve.

I find the report difficult to distill into key messages. Here are a couple paragraphs on impacts (starting on p. 1-29 if you are following along at home):

 Impacts of climate change are due to multiple environmental drivers besides rising temperatures, such as rising atmospheric CO2, shifting rainfall patterns, rising sea levels, increasing ocean acidification, and extreme events, such as floods, droughts, and heat waves (IPCC, 2014e). For example, changes in rainfall affect the hydrological cycle and water availability (Schewe et al., 2014). Several impacts depend on atmospheric composition, for example, increasing atmospheric carbon dioxide levels leading to changes in plant productivity (Forkel et al., 2016), but also to ocean acidification (Hoegh Guldberg et al., 2007). Other impacts are driven by changes in ocean heat content, for example, the destabilization of coastal ice-sheets and sea-level rise (Bindoff et al., 2007; Chen et al., 2017), whereas impacts due to heat waves depend directly on ambient air or ocean temperature (Matthews et al., 2017). Impacts can be direct, for example, coral bleaching due to ocean warming, and indirect, for example, reduced tourism due to coral bleaching. Indirect impacts can also arise from mitigation efforts such as changed agricultural management (Section 3.6.2) or remedial measures such as solar radiation modification (Section 4.3.8, Cross-Chapter Box 10 in Chapter 4).

Impacts may also be triggered by combinations of factors, including ‘impact cascades’ (Cramer et al., 2014) through secondary consequences of changed systems. Changes in agricultural water availability caused by upstream changes in glacier volume are a typical example. Recent studies also identify compound events (e.g., droughts and heat waves), that is, when impacts are induced by the combination of several climate events (AghaKouchak et al., 2014; Leonard et al., 2014; Martius et al., 2016; Zscheischler and Seneviratne, 2017).

The rest of the report goes into various scenarios and pathways for achieving the 1.5 degrees C limit.

The Summary for Policy Makers has some attempts to convey these concepts in a more graphical way.