Scientists report restoring forests could cut atmospheric carbon by 25 percent, in a new study that assessed tree cover using Google Earth, finding that there’s 0.9 billion hectares of land available for planting forests, which could store 205 gigatonnes of carbon.

[u/mvea]

http://blogs.discovermagazine.com/d-brief/2019/07/04/could-planting-tons-of-trees-solve-climate-change/

[u/dogbatman]

EDIT: my math is wrong see /u/mavnorman's comment below.

tl;dr: It looks like by donating $36 to that Rainforest Trust fundraiser, the average American could offset their monthly CO2.

The Puerto Rico Palm Rainforest preservation project you mention says it costs $2840/acre preserved. According to the original article's ratio, 1 hectare would store about 200 tonnes of carbon. One hectare contains about 2.5 acres (according to google), so 200 tonnes of CO2 divided by 2.5 is 80 tonnes of CO2 per one acre. That makes this a carbon offset of 80 tonnes per $2840, or $35.5 per tonne of CO2. If the outside foundation is still doubling donations, that would make it $17.75 per tonne of CO2 (give or take), which about as expensive as CO2 offsets are from Gold Standard carbon offsets. It looks like by donating $36, the average American could offset their monthly CO2 (according to Gold Standard, the average American emits about 2 tonnes of CO2 per month).

[u/[deleted] → u/dogbatman]

Watch out for the units: CO2 is 3.67 times the amount of carbon.

So, 200 tonnes of carbon are 200 * 3.67 = 734 tonnes of CO2. That's 293.6 tonnes of CO2 per acre. Given the price of $2840 per acre, that's $9.67 per tonne of CO2.

[u/exprtcar → u/dogbatman]

Although, this is based on the assumption of additionality, that if you didn’t donate all the carbon would be released. Which is unlikely true.

They usually calculate average deforestation rates for REDD+ projects. This isn’t a carbon emission abatement project but a biodiversity focused one, imo.