The Guardian is certainly left wing and I do read it. That article is presenting research that shows the relative impacts of different personal choices, such as having fewer children, or not running a car. It's not necessarily advocating them as serious options for fixing the problem.

The concluding statements say:

"Chris Goodall, an author on low carbon living and energy, said: “The paper usefully reminds us what matters in the fight against global warming. But in some ways it will just reinforce the suspicion of the political right that the threat of climate change is simply a cover for reducing people’s freedom to live as they want.

“Population reduction would probably reduce carbon emissions but we have many other tools for getting global warming under control,” he said. “Perhaps more importantly, cutting the number of people on the planet will take hundreds of years. Emissions reduction needs to start now.”

Seems reasonably balanced to me.