Issue 063 - Biodiversity protection - Protected-area geometry
How much of Earth must still be protected to reach "30 by 30"?
Reuters reported that countries remain behind the global 30 by 30 biodiversity target: about 20% of land and inland waters and 10% of marine areas are formally protected today.
Earth, climate, and environmentAbout 1 minute
Sources checked September 24, 2026. Figures and circumstances may have changed.
The problem
If the target is protecting 30% of Earth's land and oceans, and current formal protection is about 20% of land and 10% of marine areas, about how much additional area must be protected?
Estimate land and ocean separately, then combine them into one total area gap.
Because Fermi problems target an order of magnitude, I normally use no more than two significant digits and write most calculations in scientific notation; the Fermi reference explains both conventions.
Grounding facts
The Reuters-reported protection gap is:
land gap
~= 30% target - 20% protected
~= 10 percentage points of land
ocean gap
~= 30% target - 10% protected
~= 20 percentage points of oceanThe CIA World Factbook lists Earth's total area at about 510 million km2, with about 149 million km2 of land and about 362 million km2 of water. That is the familiar roughly 30% land, 70% ocean split.
Reuters also notes that formal designation is not the same as effective protection. The Ocean Protection Gap Report says about 10% of the ocean is designated, but only about 3.5% is assessed as effectively protected.
After checking sources
Checked answer and calculation
Start with land. The remaining target is about 10 percentage points of Earth's land area:
additional land area
~= 0.10 x 1.49 x 10^8 km2
~= 1.5 x 10^7 km2
~= 15 million km2Then do the ocean. The remaining target is about 20 percentage points of Earth's marine area:
additional ocean area
~= 0.20 x 3.62 x 10^8 km2
~= 7.2 x 10^7 km2
~= 72 million km2Combined, the remaining formal-protection gap is therefore:
total additional area
~= 1.5 x 10^7 km2 + 7.2 x 10^7 km2
~= 8.7 x 10^7 km2
~= 90 million km2That is roughly nine times the area of the United States, or roughly nine Europes. The ocean gap dominates the total: the needed additional marine protection alone is about seven United States-sized areas.
If progress had to happen evenly from late 2026 through 2030, the world would need to add on the order of 20 million km2 per year, or around 50,000 to 60,000 km2 per day. That is not a typo; percentage targets on a planet are planetary-scale commitments.
Before checking sources
Matt's original estimate
This is the unverified estimate Matt wrote before checking sources, not the checked answer.
I have a memory peg for approximate surface area of the earth that makes this one a bit easier, or shorter anyway.
I remember that the surface area of the earth is about 10^14 m2.
If the earth's surface is 70% ocean and 30% land, that means the surface area is about 7 x 10^13 m2 ocean and about 3 x 10^13 m2 land/inland water. If 20% of land and 10% of marine surface area is protected, even minimally or ineffectively, that leaves 10% and 20% further to be protected, respectively.
That ends up being about 1.4 x 10^13 m2 of ocean, or about 1.4 x 10^7 km2, and 3 x 10^12 m2 land, or about 3 x 10^6 km2.
Reasoning score
Matt's reasoning score: 70 / 100
Higher is better: earn points for useful facts, a sound reasoning approach, correct math, and a final estimate close to the sourced answer. The owl meter shows percent full of it: 100 minus the reasoning score.
Useful facts: 10/30. The land/ocean split and square-meter to square-kilometer conversion were right, but the main Earth-surface-area peg was about fivefold too low.
Reasoning approach: 30/30. The model was exactly the right one: split Earth into land and ocean, multiply each by the remaining percentage-point gap, then add the results.
Math: 10/10. The arithmetic followed correctly from the stated assumptions.
Final estimate: 20/30. The combined answer was about five times low, which is comfortably within one order of magnitude.
Post-check reflection
Matt's reflection
Looks like my mental peg was about 5x low so my answer was about the same amount low - need to update that peg. Otherwise, this is clearly a big amount of land/ocean that needs to be added to the protections.
Recommended memory peg
Remember Earth's surface area as about 5 x 10^8 km2: roughly 1.5 x 10^8 km2 land and 3.6 x 10^8 km2 ocean. That peg makes global percentage problems much less slippery.
Reader results
Bars show how submitted estimates sort into the answer choices from the gut-check prompt.