The visceral witness accounts of bush fires and floods only add to people’s awareness of the worsening climate crisis. However, these types of impacts are generally neither worse, nor happening faster, than we thought. An increase in extreme events on the scale observed was foreseen by successive scientific assessments from the Intergovernmental Panel on Climate Change (IPCC). Worsening climate change should not be mistaken for individual events that are worse than expected. Recent extremes give a clear picture of the impacts and risks that come with a human-driven warming climate. Extreme events also emphasise the urgency of action to achieve Net Zero greenhouse gas emissions and adapt to climate change, write Professor Piers Forster and Professor Corinne Le Quéré.
We are increasingly witnessing and learning more about the costs of climate change for human and natural systems. As we do, we’re building an increasingly compelling story; to limit future risks, nations must act more urgently and more decisively to reduce greenhouse emissions to zero. Yet, this does not mean that the scientific community or the Intergovernmental Panel on Climate Change (IPCC) has underestimated the pace or magnitude of climate change. In fact, looking back, the first IPCC report, published in 1990, predicted the observed rate of global warming very well .
Since the publication of that first IPCC report, we have learned a great deal more about the way the world works from careful observations and analysis, theoretical developments and improvements in our climate models. There are examples where this increased learning has led to an upwards revision of risk, most notably for future ice sheet loss and future sea-level rise – both likely to be worse than we thought. In other areas, increased observations have led to downwards revision of risk, for example with estimates of how much carbon will get emitted from the natural environment as the world warms. Overall, though, the broad emission reduction pathways laid out in the 2018 IPCC Special Report of 1.5 would avoid the most serious impacts of climate change. Adaptation measures can be informed by existing IPCC projections complemented by regional observations and projections of trends in vulnerability and exposure to risks. What is changing is that every year that passes, so the urgency of action increases and impacts become more evident. So, what can we say with confidence?
Warming is unfolding as projected. Based on the 2013 IPCC assessment report, global temperature has continued to rise. The decade 2000-2009 was about 0.2°C above the average temperature of the previous decade. This trend has continued to today. IPCC 2013 stated that climate models project near-term increases in the duration, intensity and spatial extent of heatwaves and warm spells, which is exactly what is happening. We saw multiple heatwaves in the summer of 2018 in Europe, East Asia and Australia, and a European heatwaves in June 2019 which had never been experienced so early in the season before. New temperature extremes are being recorded around the world. This is to be expected. IPCC 2013 stated that increases is droughts would be likely in some areas.
- Flooding is expected to get worse unless we adapt. In the UK, an increase in winter time extremes of rainfall was detectable ten years ago and there is a strong theoretical basis for these extremes increasing by 7% or more per degree of global warming . These changes will increase the frequency, severity and extent of flooding, for example the UK Climate Change Risk Assessment shows that by the 2050s the UK population living in areas of flood risk could have increased from 1.8 million currently to between 2.6 and 3.3 million depending on emission pathway . Worldwide, the IPCC’s 2013 report projected an increase in the frequency, intensity, and/or amount of heavy precipitation and this is what we are seeing, with devastating consequences for people in flood-hit areas as flood risk rises. Flooding is also increasing in coastal areas as climate change pushes up sea-levels . Sea-level rise is now accelerating, as predicted by the IPCC in 2013. Projections for future sea-level rise have consistently been revised upwards across successive IPCC reports as early models were known to miss important processes that would lead to further rises but had not been fully quantified.
- Risks of tipping points are clearer than ever, but not necessarily worse than expected. Assessments of what tipping points there could be in the climate system and their possible thresholds have changed very little in the past decade. However, risks of tipping points occurring at low levels of warming becomes clearer with continuous monitoring. Several ice sheets could collapse as the climate warms, and we are already seeing melting speeding up in both Greenland and the West Antarctic ice sheet. Carbon storage in the Amazon rainforest is slowing down as the warming increases , but is not slowing down as badly as was predicted by the first attempts to estimate this which suggested a possibility of complete die-back by the end of the century . The permafrost is thawing in the Arctic, but estimates of associated methane seeping out of it remain within the bounds projected by the IPCC in 2013. The slowdown of the Atlantic Overturning Circulation, the large scale ocean current that takes surface water down to depths in the North Atlantic, is also now observed  at a rate of warming roughly consistent with IPCC 2013 projections.
- Risks of low probability events received less attention than the most likely estimates of future climate change. Previous IPCC reports largely concentrated on discussing the best estimates of future change. However, it is also important to consider the low probability futures, where climate change could be worse than feared (high risk), and equally futures where climate change is not as bad as feared (low risk) and unnecessary interventions might be avoided. Risks are considered when examining future temperature projections from the latest set of climate simulations. Several of these new models show considerably more warming over this century compared with the earlier generations of these models, but one model actually exhibits less warming . Other lines of evidence from observations and theory, including how well these models represent current warming rates, suggest that predictions of high warming futures are unlikely, but we are also not able to rule them out. Therefore, they represent a risk that needs to be considered in future planning but not given undue weight.
- Evidence of climate change impacts is growing. As impacts become more easily discernible, so does the sense of crisis and urgency. For example, flooding from extreme rainfall and coastal surges associated with successive hurricane landfalls in the USA are drastic evidence of increasing impacts of a warming climate . Likewise, the Australian wildfire season that devastated the country in recent months was fuelled by the increasing severity of extreme heat and strong winds , as predicted by the Australian government in 2008 . Many impacts are yet to emerge from the day-to-day and month-to-month natural variations in the weather, with the most difficult and potentially devastating likely to be those affecting human health and biodiversity worldwide. Complex impacts are not well captured by the models. Their exact extent and power to cause damage will become clearer as they unfold.
Our Committee’s advice, for the UK to achieve Net Zero emissions by 2050 is consistent with current scientific knowledge and understanding of future risks. The best way to avoid those risks is to get to Net Zero as soon as possible as well as ensuring we are adapting to the change that is inevitable and planning for more extreme risks as part of good risk management. Government responses to climate change must demonstrate the urgency of the climate crisis and deliver a commensurate plan of action.
 (e.g. Fowler et al. 2010, doi:10.1007/s10687-010-0101-y; Boucher et al. (2013) IPCC AR5 Chapter 7, Figure 7.21 http://www.climatechange2013.org/report/reports-graphic/ch7-graphics/).
 Schaller, N., Kay, A., Lamb, R. et al. Human influence on climate in the 2014 southern England winter floods and their impacts. Nature Climate Change 6, 627–634 (2016). https://doi.org/10.1038/nclimate2927; Climate Change Risk Assessment Report (CCRA) 2017 – page 63 of synthesis report.
 IPCC Special Report on Oceans and the Cryosphere, Chapter 4, page 342.
 IPCC Special Report on Oceans and the Cryosphere, Chapter 4, pages 360 pp