Have we crossed a dangerous warming threshold? Here's what you need to know.

The planet's hottest January on record has also helped global warming reach another unwelcome milestone, according to data released Thursday by the European Union's climate monitor: Over the past 12 months, the average global temperature has been more than 1.5 degrees Celsius, or 2.7 degrees. Fahrenheit, higher than at the beginning of the industrial age.

That number has special significance in international efforts to halt dangerous climate change. Under the 2015 Paris Climate Agreement, countries agreed to try to limit global warming to 1.5 degrees Celsius above pre-industrial times, or at least keep it comfortably below 2 degrees Celsius .

The latest temperature data does not mean that we have already passed that lower limit. Still, it is a powerful symbolic reminder that, barring major changes in the climate or the global economy, we are headed in that direction in the coming years.

Here's what you need to know.

What are the 1.5 and 2 degree objectives?

It may be helpful to start with what they are not, namely thresholds encoded somewhere in the laws of nature. Instead, they represent levels of warming that would have consequences that are unacceptably difficult for societies to manage, as decided and agreed upon by the nearly 200 countries that signed the Paris Agreement.

Deadlier hot spells. Higher sea level. Greater loss of biodiversity. Longer droughts and more intense storms. Scientists agree that these and other effects of a hotter Earth would increase significantly if warming were to increase much beyond recent levels. The temperature targets therefore represent guardrails that humanity must avoid for the sake of our communities, ecosystems and landscapes.

In fact, many of these physical effects of warming are already intensifying as we continue to add heat-trapping gases to the atmosphere by burning fossil fuels. That is why scientists and diplomats often emphasize that even if we warm the planet above 1.5 degrees of warming in one day, it will still be worth trying not to keep the temperature above 1.6, or 1.7 or 1.8 to increase.

What would it mean to cross those thresholds?

The most important thing about the Paris goals is that they are long-term goals. So technically we can be sure that we will have passed them only after a certain number of years – maybe even only after a decade. Researchers say we should not declare failure when the mercury rises above 1.5 degrees for a day, a month or even twelve months.

A host of factors – the periodic climate phenomena El Niño and La Niña, volcanic eruptions, plagues and pandemics, not to mention sheer random chance – influence the planet's precise temperatures from year to year. These factors are not what the Paris goals are about.

Different climate monitoring agencies also produce slightly different estimates of how hot the planet is at any given time, depending on how they combine and analyze the mountains of meteorological data collected by satellites, sensors and weather balloons. That means the time at which we can pass those points may vary slightly depending on who is measuring.

For example, according to the European Union's Copernicus Climate Change Service, 2023 was 1.48 degrees Celsius warmer than the pre-industrial baseline. But according to Berkley Eartha research group in California, it was 1.54 degrees Celsius warmer.

How close are we to passing it?

On average over recent years, humans have caused warming of about 1.2 to 1.3 degrees Celsius since the Industrial Revolution, most estimates suggest. And based on the current rate of CO2 emissions, it will only be a few years before we have changed the chemistry of the atmosphere so much that even drastic cuts in emissions would not be enough to prevent warming from eventually exceeding exceeds 1.5 degrees.

The first official report on countries' progress toward achieving the Paris goals, issued last year, was not optimistic. Current climate pledges from governments would put the world on track to warming of about 2.5 degrees Celsius by 2100, the report said. And that's assuming countries follow through with their plans to reduce emissions, a task that is proving difficult more than eight years after the signing of the Paris Agreement.