How reef metrics are calculated

The number we show is live coral cover — the percentage of the seafloor at the dive location that is covered by living, healthy coral. The rest is rubble, sand, algae, or bleached / dead coral skeleton. It comes from in-water surveys (line- or point-intercept transects) done by reef monitoring programs.

Higher is better. As a rule of thumb, a healthy tropical reef sits around 50% live cover. Anything under 30% is a reef that has lost a lot, and under 20% is severely degraded.

We show two snapshots of the same site so you can see its trajectory, not a generic baseline:

• A decade ago — the earlier survey on file, typically from the early-to-mid 2010s.
• Today — the most recent survey on file. The year of each survey is shown under the bar.

When a reef is losing cover, we extend the line forward. The math is deliberately simple:

1. Take the difference between the two surveys (e.g. 37% → 28% = 9 points lost).
2. Divide by the years between them (e.g. 9 ÷ 10 = 0.9 points per year).
3. Divide today’s cover by that annual loss to get years until zero (e.g. 28 ÷ 0.9 ≈ 31 years).

This is a linear extrapolation, not a forecast. It assumes the recent rate continues — which it might not. A single bleaching event can collapse a reef in a season; strong protection and cool years can slow the slide. Treat the year as a “if nothing changes, you’ve got this long” signal, not a prediction.

We don’t show a projection when a reef is holding steady or recovering, or when we only have one survey on file.

The bleaching alert comes from NOAA Coral Reef Watch. Levels run from no-stress → watch → warning → alert-1 → alert-2, and roughly track how much heat the reef has absorbed (in degree-heating-weeks).

Two different things, two different cadences:

• Thermal stress (the NOAA alert level, degree heating weeks, and SST anomaly) is pulled from NOAA Coral Reef Watch nightly. The label next to each value shows the date of the most recent satellite product.
• Coral coveris a snapshot from the most recent in water survey we have on file. For well funded jurisdictions (GBR, Florida, Hawaii) that’s often within the last year or two; outside those, it can be five or ten years old. The label shows the survey date — and an age warning if it’s more than 2 years old.

The /data page lays the whole picture out — what’s live, what’s a snapshot, what we can’t see at all today.

The five levels track how much heat the reef has absorbed, measured in degree heating weeks (°C weeks) — the integral of temperature over the long term summer maximum.

• No Stress — sea surface temperature at or below the warmest monthly mean. Reef is unstressed.
• Watch — SST is above the warmest monthly mean but DHW is still ~0. Heat is starting to accumulate.
• Warning — DHW between roughly 0 and 4 °C weeks. Possible bleaching, mortality unlikely.
• Alert Level 1 — DHW ≥ 4 °C weeks. Significant bleaching expected.
• Alert Level 2 — DHW ≥ 8 °C weeks. Widespread bleaching and significant coral mortality likely.

These are categorical thresholds, not predictions. A reef shown at Alert 2 may still recover; one shown at No Stress can be hit by next month’s heatwave.

Honestly — no, not from what we have today. We can tell you:

• What the current thermal stress alert level is.
• What live coral cover was at the last in water survey.
• What that survey measured a decade earlier, when both snapshots exist.

What we can’tdefensibly say from current data: that fish populations are declining, that sharks are “disappearing”, or that a reef is on a terminal trajectory. Sightings without an effort denominator (how many divers, how many hours) don’t support trend claims, and we won’t pretend they do.

Maybe. It’s the most asked question and also the most honest answer we have: we’re thinking about it but haven’t built it.

The shape we’re exploring is targeted “citizen missions” — specific reefs where standardised before and after imagery would matter — with a small sponsored payout per qualifying photo set. None of that is live, priced, or promised. The volunteer survey space is already crowded, and we don’t want to add noise without a real brief.

Today: affiliate links to operators, lodging, and gear. If you book through one of those links, the site earns a commission. Editorial recommendations and source/methodology disclosures don’t change based on commission rates — see About → Editorial independence.

Longer term, the affiliate income is a floor, not the plan. The wedges we’re looking at are research and NGO data subscriptions, and post event evidence infrastructure for conservation funders. Neither exists yet and neither is funding the site today.