Over the past 40 years, the economic gap between the world's richest and poorest countries and peoples has narrowed sharply. Goods, services, and people began to cross borders at greater velocity, creating opportunities for people to live healthier, more secure, and more prosperous lives. Billions rose out of poverty.
Here's the catch: all that beneficial economic activity has also sharply increased the use of fossil fuels and the amount of carbon that's reaching the atmosphere. Life on earth remains possible only because carbon dioxide in our atmosphere captures enough heat from the sun to sustain us while deflecting enough extra heat into space to keep us from burning up. That's the "natural greenhouse effect."