In 2021, the U.S. Border Patrol has apprehended 900,000 people at the border with Mexico. That’s more than twice as many apprehensions as the agency made last year, and the available data for 2021 is only through May, less than half of the year. The number of apprehensions over the past five months is even larger than the annual total in 2019, a year during which a surge in border crossings provided the rationale for President Donald Trump to declare a national emergency and build his promised wall on the border.

Soon after President Biden took office, this surge in border crossings became a political crisis. News coverage showed children crammed into temporary detainment areas, and Republicans blamed Biden for encouraging their arrival. As the months passed, that criticism waned, even as the overall number of apprehensions didn’t change much.

There’s a reason for that. While there have been far more apprehensions this year than in years past, it matters who is being detained. It’s the evolution of who’s crossing that has the most significant effects on the country’s ability to deal with the surge.

To give a sense of scale, we took monthly data since January 2016, depicting every 1,000 apprehensions as a small square. This is how the situation at the border has evolved over the past 65 months.

We flagged two particular significant periods. The first was the surge in arrivals that peaked in 2019. The second marks the point at which the U.S.-Mexico border was closed as the coronavirus began to spread.

We labeled the 2019 period as “family surge,” for reasons made obvious by our color scheme. Thousands of parents arrived at the border with their children, many of them seeking asylum within the United States. As the increase began the previous year, the Trump administration had implemented a policy of separating children from their parents upon arrival, in hopes that breaking up those families would serve as a deterrent. Over the short term, it didn’t.

But, again, it’s who was arriving that was important. More families meant more children — and more migrants subject to more resource-intensive processing systems. There are limits on how long children can be detained, the sorts of facilities in which that detention can take place and what services the children are provided. Handling a big influx of adults is one thing. Handling a big influx of children is another thing entirely.

That was the problem at the beginning of 2021, too. The media images of detained children emerged as the Biden administration was trying to figure out how to handle things. The combination of an increase in children arriving by themselves and a new surge in families meant an unusual number of minors to be addressed. As the months passed, the administration got a better handle on the problem — and the numbers of migrants in those groups declined.

What has surged is unaccompanied adults, a group that’s much less onerous to handle.

The recent moments of crisis have emerged when two factors overlap: more migrants and a higher density of children among them. For now, that latter factor is waning.

At some point, it’s likely the political narrative about migration will shift to the fact that so many migrants are being apprehended. The monthly numbers are, in fact, higher than they have been at any point in two decades. That surge, though, is a function of individual adults, and it’s a surge that began last summer, making it far harder to blame on Biden.

The politics of immigration are complicated and always evolving. For the moment, though, the president and his team can probably afford at least one exhalation.