399.68 ppm was the May 4 daily average reading for atmospheric carbon dioxide. The Keeling Curve: A Daily record of Atmospheric Carbon Dioxide from Scripps Institution of Oceanography at UC San Diego posts the latest reading along with various historical charts to help in understanding the significance of the current number.
The chart for the past week shows that some hourly readings already had reached 400 ppm.
It may well be that this week, with the "Do you want to be made well?" question from Sunday's Gospel (John 5:1-9) still fresh in our thoughts, that we will hit 400 ppm for a daily average.
The Keeling Curve website includes a page describing What Does 400 ppm Look Like? . The last time carbon dioxide levels were this high was during the Pliocene period (3 to 5 million years ago). This is the first time in human history that carbon dioxide levels have been this high; we have changed our biosphere in a way we are only beginning to understand. What we know about temperatures and sea levels in the Pliocene period can help us understand what we may be experiencing.
Here's a spiritual exercise for today: Ground yourself in prayer and Christian hope and take a look at what 400 ppm looks like, then decide your answer to the question: "Do you want to be made well?"