Not primary, but deforestation is still significant. From Scientific American:

"By most accounts, deforestation in tropical rainforests adds more carbon dioxide to the atmosphere than the sum total of cars and trucks on the world’s roads. According to the World Carfree Network (WCN), cars and trucks account for about 14 percent of global carbon emissions, while most analysts attribute upwards of 15 percent to deforestation."

https://www.scientificamerican.com/article/deforestation-and...

Some back of napkin numbers:

Rainforests store 250 tons carbon/hectare

The Amazon is 550,000,000 hectares in area

The Amazon has 137 gigatons of C total

Since 1850, humans have released around 1500 gigatons.

In particular, recently from 2000 to 2015, around 500 gigatons have been released by human activity.

Around 2700 gigatons remain in the ground.

All of these statements are just random numbers I found off the Internet, so take them with a gigaton of sodium.

> the Amazon as a whole only absorbs two or three times the carbon emissions as the UK.

The Amazon probably doesn't absorb any net amount of carbon, unless it's growing.

Let's demystify this: The carbon trapped in the Amazon is the plants. When plants consume CO₂, the C part becomes the plant. In a forest in equilibrium, growing plants do capture carbon from the air, but rotting dead plants release an equal amount.

So when you chop down a forest, and for the sake of example burn all the plants, you're releasing a large one-time amount of CO₂, but then the whole damage is done. And if you then leave the area alone, the forest will regenerate over the next 50-100 years, reabsorb the same amount of CO₂.

If you chop down the forest and build lasting structures from the wood, you're actually removing net CO₂!

I wonder if we could come up with some sort of fast growing plant that we could then use as a replacement for sand as a filler. This would reduce the sand crisis and capture carbon, if it's possible.

There is a sand crisis too?

Is it the one caused by dams?

Between 2011 and 2015 China poured more concrete than the US did in the entire 20th century.

Sand is one component of most concrete.

https://www.theguardian.com/cities/2017/feb/27/sand-mining-g...

See https://news.ycombinator.com/item?id=13789108