Because we are one species with one destiny on a path to our own destruction.
The human race is all one species living on a planet with limited space and limited resources. There is a limit to the clean air we can breath, clean water we can drink, and food we can eat. There is a limit to the amount of heat we can pump into the air before it becomes to hot to sustain life because there is a limit to how fast the earth can dissipate that heat. Global warming is real and a threat to our survival because we have exceeded the natural limits of our environment.
No matter where we live, no matter what color our skin, no matter what belief system we follow, no matter what political system we live under, we are one people with one destiny.