You may be thinking that 1.5 degrees isn't that much. Because sure, whether it's 30° or 31.5° on any given day doesn't make much of any difference and you probably wouldn't even notice.
However, it's about global averages. The average temperature of Earth as a planet is the average of all temperatures measured everywhere on Earth near ground level on 365 days a year. If that average changes, it's because a large number of individual data points have changed significantly. While in some places and on some days it may have been colder than the year before, in most places and on most days, it was warmer. Not a lot warmer, the differences year-to-year are small. But decade by decade, the changes should average out - which they don't anymore.
There are a lot of heat sinks on Earth, most significantly the oceans. They absorb huge amounts of energy which is used by the lifeforms living in the oceans (plankton). This ecosystem is attuned to a certain amount of energy coming in from the Sun. It has evolved to use that energy in the most efficient way and thus to keep the temperature of the water steady. It takes vast amounts of energy to heat them up significantly because they are of course massive bodies of water. In the past, when natural causes changed the amount energy, it was usually at a rate slow enough for the ecosystem to keep up and adapt. So if you measure the average temperatures of the oceans (or rather temperatures just above sea level) and you see a rise, something is out of the ordinary. It means that there is more energy going into the oceans than they can absorb and the change is happening too fast for the ecosystem (and evolution!) to keep up with.
I'm no climate scientist so I can't tell you why the numbers are these exact numbers (like why is the absolute upper limit 3 degrees and not 4). But there are by now some quite sophisticated climate change models based on decades of climate research and huge amounts of data about past changes (going back not only decades but millennia). These models can simulate different outcomes for different future scenarios. They're not infallible of course and they are man-made. But these models are used to predict what would happen if temperatures increase by different amounts and that's where the upper limits come from.
lumidaub t1_iuidvw9 wrote
Reply to ELI5 why is everybody saying that an 1.5°C increase in global temperature is catastrofic? by BloodyBite1
You may be thinking that 1.5 degrees isn't that much. Because sure, whether it's 30° or 31.5° on any given day doesn't make much of any difference and you probably wouldn't even notice.
However, it's about global averages. The average temperature of Earth as a planet is the average of all temperatures measured everywhere on Earth near ground level on 365 days a year. If that average changes, it's because a large number of individual data points have changed significantly. While in some places and on some days it may have been colder than the year before, in most places and on most days, it was warmer. Not a lot warmer, the differences year-to-year are small. But decade by decade, the changes should average out - which they don't anymore.
There are a lot of heat sinks on Earth, most significantly the oceans. They absorb huge amounts of energy which is used by the lifeforms living in the oceans (plankton). This ecosystem is attuned to a certain amount of energy coming in from the Sun. It has evolved to use that energy in the most efficient way and thus to keep the temperature of the water steady. It takes vast amounts of energy to heat them up significantly because they are of course massive bodies of water. In the past, when natural causes changed the amount energy, it was usually at a rate slow enough for the ecosystem to keep up and adapt. So if you measure the average temperatures of the oceans (or rather temperatures just above sea level) and you see a rise, something is out of the ordinary. It means that there is more energy going into the oceans than they can absorb and the change is happening too fast for the ecosystem (and evolution!) to keep up with.
I'm no climate scientist so I can't tell you why the numbers are these exact numbers (like why is the absolute upper limit 3 degrees and not 4). But there are by now some quite sophisticated climate change models based on decades of climate research and huge amounts of data about past changes (going back not only decades but millennia). These models can simulate different outcomes for different future scenarios. They're not infallible of course and they are man-made. But these models are used to predict what would happen if temperatures increase by different amounts and that's where the upper limits come from.