Record Increase

Although carbon dioxide emissions have been stable, the atmospheric concentration reached 403.3 ppm according to the World Meteorological Organization.

The record increase of 3.3 ppm in CO2 from 2015 to 2016 was larger than the previous record increase, observed from 2012 to 2013, and the average growth rate over the last decade.

Update:  A study published in The Lancet finds that climate change is already having a major impact on health.

[H]eatwaves over the past two decades were hotter and lasted longer, vector-borne diseases increased as warmer temperatures spread insects, and allergies worsened as unseasonable weather prolonged exposure to pollen.

Update (November 6):  I'm not sure if there is a connection to the ppm increase, but a study published in Science documents the impact of degraded tropical forests.

Researchers found that forest areas in South America, Africa and Asia – which have until recently played a key role in absorbing greenhouse gases – are now releasing 425 teragrams of carbon annually, which is more than all the traffic in the United States.

Update (November 26, 2019):  The World Meteorological Organization announced a carbon dioxide concentration of 407.8 ppm for 2018 following 405.5 ppm for 2017. WMO Secretary-General Petteri Taalas:

It is worth recalling that the last time the Earth experienced a comparable concentration of CO2 was 3-5 million years ago. Back then, the temperature was 2 to 3 degrees [centigrade] warmer, [and] sea level was 10 to 20 meters higher than now.