It seems particulate matter was reducing the impact of increasing CO2. The rate of warming increased as countries around the world cleaned up their air.

There are a number of factors driving the acceleration of warming. While the world has made real progress in slowing down the growth of carbon dioxide and other greenhouse gas emissions, they have yet to peak and decline. And on top of this, we are reaping the results of what the climate scientist James Hansen calls our “Faustian bargain” with air pollution. For decades, air pollution from sulfur dioxide and other hazardous substances in fossil fuels has had a strong temporary cooling effect on our climate. But as countries around the world have begun to clean up the air, the cooling effect provided by these aerosols has fallen by around 30 percent since 2000. Aerosols have fallen even more in the past three years, after a decision to largely phase out sulfur in marine fuels in 2020. These reductions in pollution on top of continued increases in atmospheric greenhouse gas concentrations mean that we are encountering some of the unvarnished force of climate change for the first time.

https://www.nytimes.com/2023/10/13/opinion/climate-change-excessive-heat-2023.html?unlocked_article_code=MV7Te-BwrX01dDW9YN2x5i4liJ_txffN-7pATpfGL_WC8zVhXtr_JU-6g0GzGtHkwJeXVpDva6LsJulzgYNYhtdocm3bYx2f2XAZ0iWJjBCyuN4rCK-ilrHccUS9yD90VAfe156hv2dsKig2GNAQPOhRtemftQpsWoEH2Do5wbpAOsqFKLoslF2XMTbUm78egNN-0FWwIBPLwOENIXjyug_r5oR-nhVVtAf-8J0wx5YrL8AWd4rg7GXODDSQDCdU8evFlOL5djqlpOaS-AdjgmdK5m_CHhrblDdn1-GI1Rc4TKJhiSOLP-2Vmu-0YGPWZmyWiqnVV09PU3MubJzdXWI09rKmvYW5dvsL