1.5°C: What’s the Big Deal?




In 2015, the parties to the United Nations Framework Convention on Climate Change (UNFCCC) asked the world’s foremost scientific body on the subject, the Intergovernmental Panel on Climate Change (IPCC), to create a special report on limiting warming to 1.5°C. In October, 2018, after three years of work and the consultation of over 6,000 scientific articles, the 91 lead authors of the report published their findings. The main takeaway from the report is that, despite the global community’s lackluster response to climate change thus far, there is still a window for us to limit warming to 1.5°C—albeit, a very narrow window.

Limiting warming to 1.5°C will be anything but easy. It will almost certainly require significant research, development, and upscaling of negative emissions technologies—technologies that are costly and, as of yet, unproven. It will also require us to reduce our emissions at rates never before seen in history. For instance, following the IPCC report, emissions will have to fall to 45% of what they were in 2010 by 2030.[1] According to the World Bank, carbon dioxide emissions in 2010 totaled 33.5 Gt. This means we need our global carbon dioxide emissions to be at roughly 15 Gt by 2030. In 2017, we emitted roughly 36.8 Gt of carbon dioxide, and our emissions for 2018 may be even higher. Given our current global emissions, the yearly reductions between now and 2030 are steep, to say the least. Emissions need to decrease by about 7-8% each year in between now and 2030. Not only that, but we’ll have to reach net zero emissions by 2050. Make no mistake about it, limiting climate change to 1.5°C will be extremely burdensome and will require significant global cooperation.

With such a burdensome path in between us and limiting warming to 1.5°C, one might wonder: is 1.5°C really that much better than the goal of 2°C? Should we really aim for the lower target? The answer according to the IPCC report is a resounding yes.

The difference between 1.5°C and 2°C—or the difference between 2.7°F and 3.6°F—may not sound like much. One might think: what’s the big difference between a summer day of 33.5°C and a summer day of 34°C? Unfortunately, that half a degree makes a huge difference when it accumulates across the entire global surface area. In fact, according to the IPCC, the difference between 1.5°C and 2°C will be robust. In the remainder of this article, I want to highlight 10 of the most pressing concerns raised in the IPCC’s report.

1. Droughts

Risks from droughts and precipitation deficits are projected to be higher at 2°C compared to 1.5°C global warming[2]

Droughts are one of the most devastating effects of climate change. With negative effects on the availability of drinking water, agricultural outputs, and biodiversity, nearly every region of the world has reason to be concerned. But what comes out of the IPCC report is that the Mediterranean region and southern Africa will be hit the hardest if we let warming exceed 1.5°C. Indeed, multiple authors have argued that it was the persistent drought that had negative impacts on the Euphrates basin.[3] This in turn played a significant role in the prolonged conflict in Syria, a conflict that has displaced millions of people. Extended droughts are one of the main climatic impacts that will unfortunately create large numbers of climate refugees in the years to come. And these droughts will be more frequent and more harmful at 2°C than at 1.5°C.

2. Heavy Rain and Storms

Heavy precipitation associated with tropical cyclones is projected to be higher at 2°C compared to 1.5°C global warming [4]

Of course, too much water is also a problem. Heavy rains and more intense storms often lead to flooding, wreaking havoc on communities that have been unable to adequately adapt beforehand. Perhaps one of the regions most at risk, the Brahmaputra River in Bangladesh will experience more frequent and more devastating flooding with warming of 2°C compared to that of 1.5°C. The people living in this flood plane in Bangladesh are some of the poorest people on the planet, and intense flooding there would have a regrettably negative impact.

3. Sea-Level Rise

Risks associated with sea level rise are higher at 2°C compared to 1.5°C [5]

Another impact of climate change that is catastrophic for low-lying islands and coastal cities is sea-level rise. Sea-level rise will almost certainly be less pronounced with warming of 1.5°C compared to that of 2°C. But even more important is the rate at which the waters encroach. “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.”[6] There is high confidence that sea-level rise will continue even after emissions have reached net zero.[7] But to the extent that we can slow the rate at which sea-levels rise, we can give at risk communities more time to adapt and prepare for the imminent changes.

4. Forest Fires and Biodiversity

Impacts associated with … biodiversity-related risks such as forest fires, and the spread of invasive species, are lower at 1.5°C compared to 2°C of global warming [8]

With the forest fires that have been ripping through California the last couple years, climate change is no longer seen as a threat looming in the future. The effects of a warmed world are hitting here and now, and in a visible and terrifying way. While we know that forest fires will become more frequent and more destructive for human settlements, it is also important to note the devastating impacts they have on biodiversity. Biodiversity is an important health indicator for the planet, and what’s clear form the IPCC report is that biodiversity will suffer greater losses at 2°C than at 1.5°C.

5. Coral Reefs

Coral reefs, for example, are projected to decline by a further 70–90% at 1.5°C (high confidence) with larger losses (>99%) at 2ºC [9]

Our previous emissions have already caused the globe to warm by roughly 1°C. Such warming is already having measurable impacts on the health of our oceans. For example, with only 1°C of warming, we’ve already lost roughly 50% of our coral reefs. And the projections for future warming are hard to believe. The thought of losing 70-90% of our coral reefs may make us wonder why 1.5°C is even worth aiming for. But when we learn that more than 99% of our reefs will be lost with warming of 2°C, the 10-30% we could save by limiting warming to 1.5°C immediately becomes worthwhile.

6. Heat-related Morbidity

Lower risks are projected at 1.5°C than at 2°C for heat-related morbidity and mortality [10]

Due to regional differences such as access to air conditioning, historical temperatures, and population vulnerability, some regions of the world will experience greater increases in heat-related morbidity and mortality than others. There will, of course, be some decreases of cold-related mortality, but they are generally projected to be offset by the increases in heat-related mortality. The most at-risk parts of our population are children, older adults, and women, and their exposure to risk will almost certainly be higher at 2°C than at it would be at 1.5°C of warming.

7. Vector-Borne Diseases

Risks from some vector-borne diseases, such as malaria and dengue fever, are projected to increase with warming from 1.5°C to 2°C [11]

Diseases like malaria and dengue are transmitted through vectors, most commonly mosquitoes. Mosquitoes can only breed in particular habitats, and, unfortunately, climate change is expected to expand those habitats suitable for mosquitoes and other disease vectors. To take but one example, malaria kills nearly half a million people every year, most of whom are children under the age of 5, the vast majority of whom are living in sub-Saharan Africa. Even a 10% increase in malaria cases could mean an extra 50,000 deaths each year—again, the majority of which will be children under the age of 5. And this is only malaria. Dengue, chikungunya, yellow fever, and zika will also most likely see increases, with corresponding negative impacts on human health.

8. Crop Yields

Limiting warming to 1.5°C, compared with 2ºC, is projected to result in smaller net reductions in yields of maize, rice, wheat, and potentially other cereal crops [12]

As of 2018, the global population has reached 7.7 billion. The United Nations predicts us to hit nearly 10 billion by 2050 and over 11 billion by 2100.[13] This roughly 50% increase in global population, coupled with greater average wealth through the globe, will carry with it a significant increase in demand for food. As with most impacts from climate change, impacts on crop yields will vary region to region. But what is certain is that warming will have adverse effects on crop yields and crop nutrition in the aggregate, and that lower latitudes (where much of the world’s poorest populations live) will be hit the hardest. And, as is summarized by the quote above, warming of 2°C will be even worse than warming of 1.5°C

9. Water Stress

Depending on… socioeconomic conditions, limiting global warming to 1.5°C… may reduce the proportion of the world population exposed to…water stress by up to 50%[14]

Droughts, desertification, and sea-level rise all contribute to water stress. Water stress describes situations in which the demand for water exceeds the supply of water during a particular time period, resulting in significant harms both to human health and the economy. While water stress is likely to threaten communities across the globe, it is sub-Saharan Africa and South Asia that will be hit the hardest. If we are able to limit warming to 1.5°C, we can reduce the proportion of the world exposed to water stress by 50%. This in and of itself is a significant reason to aim for 1.5°C as opposed to 2°C.

10. Economic Losses

Risks to global aggregated economic growth due to climate change impacts are projected to be lower at 1.5°C than at 2°C by the end of this century[15]

Climate change carries a double economic punch. Not only are damages from climate change expected to cost hundreds of billions to perhaps 1 trillion USD per year, but climate change will also have a negative effect on global GDP. It is awfully difficult to come up with accurate numbers for economic damages and losses to GDP. This difficulty lies in estimating the social cost of carbon and determining an appropriate discount rate. Setting these difficulties aside, the IPCC reports that limiting climate change to 2°C will still allow roughly 69 trillion USD in economic damages by 2100. Limiting climate change to 1.5°C will carry with it economic damages in the order of 54 trillion USD.[16] Thus, that half of a degree difference translates into something like 15 trillion USD.



What should we conclude from all of this? It should be noted that the IPCC report does not endorse any particular temperature target. Rather, the IPCC simply laid out what very well may happen at different warming levels. Whether to aim for 1.5°C, 2.0°C or any other temperature target is not something that scientists alone can decide. There are costs, benefits, and tradeoffs associated with any target, and deciding between those costs, benefits, and tradeoffs requires a value judgement that only the international community as a whole can make. But what should be clear from the previous paragraphs is that the negative effects associated with higher temperature targets are staggering, and that we would need weighty reasons to allow warming that would engender such negative effects.



Now, it should also be noted that these impacts will not fall evenly across the entire globe. That is, while some regions may experience drought, others may experience heavy precipitation. And while some regions may experience significant crop losses, for others the warmer weather may actually correlate with crop yield increases. This variation notwithstanding, what is certain is that disadvantaged populations throughout the globe will be disproportionately harmed by warming above 1.5°C. Poorer populations, indigenous peoples, and communities dependent upon agricultural or fishing outputs will be hit hardest by climate change, especially climate change associated with warming greater than 1.5°C.[17] This is perhaps the greatest injustice associated with climate change: those who have contributed the least to the global problem will be most adversely affected.


With this injustice in mind, coupled with the exacerbated impacts predicted for all populations with warming above 1.5°C, it behooves the international community to double down on climate efforts. The science is clear—it’s time for our resolve and action to follow suit.



[1] IPCC 1.5 report SPM p. 15

[2] IPCC 1.5 report SPM p. 9

[3] IPCC 1.5 Report p. 200.

[4] IPCC 1.5 report SPM p. 9

[5] IPCC 1.5 report SPM p. 9

[6] IPCC 1.5 report SPM p. 9

[7] IPCC 1.5 Report p. 207.

[8] IPCC 1.5 report SPM p. 10

[9] IPCC 1.5 report SPM p. 10

[10] IPCC 1.5 report SPM p. 11

[11] IPCC 1.5 report SPM p. 11

[12] IPCC 1.5 report SPM p. 11

[13] United Nations, World Population Prospects 2017

[14] IPCC 1.5 report SPM p. 11

[15] IPCC 1.5 report SPM p. 12

[16] IPCC 1.5 degree report p. 264

[17] IPCC 1.5 report SPM p. 11

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About Daniel Callies

Daniel Callies

Daniel Edward Callies is a Research Fellow and PhD candidate in philosophy at Goethe University Frankfurt and heads A-ID’s research cluster on “Energy and the Environment.” Previously, he was a Predoctoral Research Fellow at Harvard University’s Kennedy School in the Belfer Center’s Science, Technology, and Public Policy Program (2016-2017), and the Bernheim Postdoctoral Fellow in Social Responsibility at the Université catholique de Louvain (2017). His research generally focuses on normative and applied ethics, global justice, and climate justice. His dissertation addressed moral and political concerns surrounding climate change — specifically, climate engineering technologies. Before coming to Europe, he studied philosophy at San Diego State University where he was awarded his BA in 2008 and his MA in 2012. When not engaged in research, he can be found surfing the many beaches on offer in Southern California.

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