CLIMATE BYTES: The Significance of 1.5 Degrees


This is the first of a series of short notes about aspects of the climate crisis. 


In 2015, the countries attending the IPCC (Intergovernmental Panel on Climate Change) meetings in Paris agreed that we need to keep our global average surface temperature below 1.5oC above pre-industrial temperatures.  This note is to give some brief context for that goal but there is much more to be said.

There is general but not total acceptance that human-caused global warming began in the last half of the 19th century when, using energy from burning fossil fuels, we changed from being an agricultural society to an increasingly industrial society. As we burned fossil fuels, the concentration of heat-trapping CO2 in the atmosphere increased. Most of the change has happened since 1950. Since the 1940s, humanity has burned about 90% of all fossil fuels ever burned: more than half of that has happened just in the last 30 years, which is also the period of the most dramatic increases in global temperature [Prof. William Rees, various Youtube lectures]. So, when it is reported [NOAA,]that the global temperature in 2023 was 1.35oC above pre-industrial levels, that is how much the earth’s average temperature has risen since industrial activity began affecting climate change.

Readers may be aware of reports that in each of the last 12 months, the earth’s average temperature reached or exceeded 1.5oC above pre-industrial levels. How does this info tally with our saying that global temperature increase in 2023 was just 1.35o C. That is because annual temperature changes are averaged over longer periods  such as 10 to 20 years. Because the 2023 temperature monthly rises are probably an El Nino effect, once we cycle back to an El Nina phase, monthly temperatures may drop below 1.5oC. 

Even so, a 1.35oC change, in the perspective of geologic time, is extraordinary. The Holocene is that 11,000-year geologic era since the last ice age. It is the period during which humans turned from being hunter-gatherers to plant and animal domesticators and, eventually, creators of our human civilization. The Holocene provided the climate stability that made it possible for humanity to transform so remarkably: throughout the period the earth’s temperature, though sometimes hotter and sometimes colder, never strayed more than 0.5oC higher or lower than 14oC [Johan Rockstrom, 44thMacaulay Lecture]. This was the earth that we now think of as the self-regulating Gaia. 

The 1.35oC annual change we are experiencing is more than two orders of magnitude greater than any change humans ever experienced during the Holocene. This is not surprising when we consider the scientific finding that CO2 concentration in our atmosphere is now the highest in the last 2 million years. And it is now clear that the speed of the earth’s temperature rise is increasing [NOAA,].

The number and severity of climate-related catastrophes in the world is also rising. At 1.35 degrees, we are already experiencing impacts such as extreme heat, more and stronger hurricanes, drought and forest fires. The 2016 Paris Accord stipulated the 1.5oC limit because, if that limit is exceeded, the increasing frequency and intensity of such catastrophic events would likely become intolerable. More than that, scientists have determined that as the earth’s temperature rises, some of its results are likely to reach a tipping point, a point beyond which changes become self-perpetuating [Tim Lenton, Tipping Points in Comlex Systems]. One example: ice cover in the earth’s polar regions has been melting. Reduced ice cover means that less of the sun’s energy is reflected back to space and more is absorbed by and generates warmth in darker open water thereby furthering ice melting. Underlining the imperative to reduce CO2  emissions, there are several other tipping points that have an increased likelihood of being triggered if long term global warming were to exceed 1.5oC. We don’t want to go there!


Author: TIG