The world will likely breach the internationally agreed-upon climate change threshold in about a decade, and keep heating to break through a next warming limit around mid-century even with big pollution cuts, artificial intelligence predicts in a new study that's more pessimistic than previous modeling. The study in Monday's journal Proceedings of the National Academy of Sciences reignites a debate on whether it's still possible to limit global warming to 1.5 degrees Celsius, as called for in the 2015 Paris climate agreement, to minimize the most damaging effects of climate change. The world has already warmed 1.1 or 1.2 degrees since pre-industrial times, or the mid-19th century, scientists say. Two climate scientists using machine learning calculated that Earth will surpass the 1.5 degree (2.7 degrees Fahrenheit) mark between 2033 and 2035. Their results fit with other, more conventional methods of predicting when Earth will break the mark, though with a bit more precision.

The world faces a significant risk of passing a crucial global warming threshold earlier than scientists had suggested, possibly as soon as 2050, a paper published Monday found. The threshold is the point at which Earth's overall temperature has increased by 2.0 degrees Celsius, or 3.6 degrees Fahrenheit. If greenhouse gas emissions remain at high levels, there's a 50% probability the world could reach that catastrophic milestone before 2050, said co-author Noah Diffenbaugh, a climate scientist at Stanford University. The chance it could reach 2.0 degrees before 2058 is 84 in 100. Earlier estimates had put it closer to the end of the century.

Records of global temperatures over the last few decades figure prominently in the debate over the climate effects of CO2 emitted by burning fossil fuels, as I discussed in my first post in this series, on What can global temperature data tell us? One recent controversy has been whether or not there has been a pause' (also referred to as a hiatus') in global warming over the last 15 to 20 years, or at least a slowdown' in the rate of warming, a question that I considered in my second post, on Has there been a pause' in global warming? As I discussed in that post, the significance of a pause in warming since around 2000, after a period of warming from about 1970 to 2000, would be to show that whatever the warming effect of CO2, other factors influencing temperatures can be large enough to counteract its effect, and hence, conversely, that such factors could also be capable of enhancing a warming trend (eg, from 1970 to 2000), perhaps giving a misleading impression that the effect of CO2 is larger than it actually is. To phrase this more technically, a pause, or substantial slowdown, in global warming would be evidence that there is a substantial degree of positive autocorrelation in global temperatures, which has the effect of rendering conclusions from apparent temperature trends more uncertain. Whether you see a pause in global temperatures may depend on which series of temperature measurements you look at, and there is controversy about which temperature series is most reliable. In my previous post, I concluded that even when looking at the satellite temperature data, for which a pause seems most visually evident, one can't conclude definitely that the trend in yearly average temperature actually slowed (ignoring short-term variation) in 2001 through 2014 compared to the period 1979 to 2000, though there is also no definite indication that the trend has not been zero in recent years. Of course, I'm not the only one to have looked at the evidence for a pause.