Less than two years ago, the leader of the free world had high hopes for curbing the harmful effects of climate change.

"[S]omeday, we may see this as the moment that we finally decided to save our planet,” then-US president Barack Obama said upon the US and China formally entering the Paris Agreement.

Today, the prospects are dismal for meeting the goals of that global accord. Last year, the US announced its intention to withdraw from the agreement, and researchers have said current country pledges are not enough to meet the goal of keeping the global temperature rise below 2-degrees Celsius.

In the August issue of the American Economic Journal: Economic Policy, Yale economist William Nordhaus revises an earlier model to project climate change in an era of minimal environmental policies.

Figure 2 from Nordhaus (2018)

The figure above shows emissions projections in four different scenarios. The first is the business as usual approach (represented by the pink triangles), where nothing is done and we have no climate policy. Then there is the “cost-benefit economic optimum” (blue circles), which optimizes current climate policy indefinitely. The two more aggressive approaches are limiting the long-term temperature increase to 2.5 degrees Celsius (green triangles) and a “Stern” policy (red stars) that takes immediate and aggressive steps in order to preserve our current quality of life for future generations (red stars).

If things proceed business as usual, then emissions continue to rise substantially. The two ambitious paths require zero CO2 emissions by midcentury, a reduction far sharper than the policies allow for in any major region. The optimal policy would have a flat trajectory over the next 50 years.

In line with past estimates, Nordhaus predicts rapid climate change over the next century if dramatic steps are not taken. He says that it is unlikely that nations will achieve the 2-degree Celsius target, even if they adopt aggressive policies in the next few years.