Coral bleaching occurs when corals are stressed and expel the colorful zooxanthellae algae living in their tissues. Without these algae, the coral's white calcium carbonate skeleton becomes visible through its transparent tissue - making the coral appear "bleached."
Vibrant colors from zooxanthellae
White skeleton visible through tissue
Just 1-2°C above normal for extended periods
Excessive UV radiation compounds heat stress
Heavy rainfall can dilute seawater
Pollution and sediment stress corals
NASA and NOAA satellites orbit Earth continuously, carrying sensors that measure ocean properties from space. The most commonly used satellite for reef monitoring is MODIS (Moderate Resolution Imaging Spectroradiometer), which orbits Earth twice daily and collects data every 1-2 days over the same location.
MODIS carries sensors that detect light reflected from the ocean surface and infrared heat radiating from the water. The satellite measures this information in specific wavelengths of light called "bands." Different wavelengths reveal different information: visible light bands show what a camera would see, infrared bands measure temperature, and other bands detect chlorophyll and other properties.
The satellite data arrives at ground stations as raw numbers—thousands of pixels, each containing measurements in multiple bands. Scientists then process this raw data into usable products: maps showing sea surface temperature, maps showing ocean color, or classified maps highlighting bleaching risk. A single satellite image might cover an area thousands of kilometers across but also contain enough detail to see individual reef structures.
The resolution of satellite imagery varies. MODIS pixels are roughly 250-1000 meters across—large enough to see broad reef systems and regional patterns but too coarse to see individual coral colonies. Finer-resolution satellites like Landsat have 30-meter pixels, sufficient to see individual reef structures, but they pass over the same location less frequently (every 16 days). Scientists choose satellite products based on their question: broad monitoring of large reef systems uses coarse-resolution daily data, while detailed assessment of specific reefs uses finer-resolution data that's available less frequently.
NOAA produces the Coral Reef Watch dataset, a global monitoring product specifically designed to detect coral bleaching risk. The dataset combines satellite sea surface temperature data with historical climate data to produce bleaching alert products available at a 5-kilometer resolution covering all the world's coral reefs.
NOAA calculates the "Degree Heating Weeks" metric for every 5-kilometer location where coral reefs exist. This metric accumulates thermal stress over time, weighting recent heating more heavily than older heating. One Degree Heating Week (1 DHW) represents one week where the sea surface temperature was 1°C above the long-term maximum monthly average for that location.
Here's the key insight: corals have adapted to the normal temperature range for their location. A 1°C increase in water temperature is a bigger stress in the stable tropical Pacific than the seasonal 15°C variation that corals in temperate waters experience. NOAA's metric accounts for this by comparing current temperatures to historical maximums, not to absolute values. A reef in the Caribbean is compared to Caribbean historical data; a reef in the South Pacific is compared to South Pacific data.
NOAA classifies bleaching risk into categories based on DHW thresholds: - 0-1 DHW: No thermal stress - 1-4 DHW: Watch level (bleaching possible) - 4-8 DHW: Alert level (significant bleaching likely) - 8+ DHW: High alert level (severe bleaching and mortality likely)
These thresholds are derived from decades of observations correlating thermal stress metrics with field observations of actual bleaching on reefs worldwide. They represent the synthesis of marine scientists' knowledge about coral tolerance to heat stress.
Let's walk through a concrete example. Suppose a reef location has a long-term maximum monthly average temperature of 29°C (the warmest month is typically August with a 29°C average). In early July, the temperature rises to 30°C and stays elevated.
In the first week of this elevated temperature, the location accumulates 1 DHW (one week at 1°C above normal). In the second week, if the temperature stays at 30°C, it accumulates another 1 DHW, for a total of 2 DHW. If the temperature rises to 31°C in week three, that week contributes 2 DHW (2°C above normal for one week), bringing the total to 4 DHW.
The important detail is that DHW is cumulative, but it "ages out." NOAA's calculation method gives more weight to recent weeks. If a location experiences 4 DHW of heating and then temperatures cool back to normal, the DHW value starts declining as the older weeks in the calculation get replaced with normal weeks. This reflects the biological reality that corals recover from brief heat stress but are increasingly damaged by prolonged stress.
When a location reaches 4 DHW, bleaching is typically observed in the field within 1-2 weeks. At 8 DHW, widespread death is common. But individual reefs vary—some reefs experience significant bleaching at lower DHW values, while others are more heat-tolerant. Scientists use DHW as a first-pass indicator, then validate predictions with field surveys and underwater imagery.
To understand satellite data in practice, you'll map bleaching risk for a reef region using actual NOAA Coral Reef Watch data.
Data source: Download the weekly DHW dataset for a recent month covering the Caribbean or Indo-Pacific (your teacher will provide a specific file or link).
Task 1 - Load and visualize: Open the dataset in a spreadsheet or mapping tool. Each row represents a 5-km grid cell; columns include latitude, longitude, and current DHW value. Create a map showing DHW values as a color gradient: blue for 0-1 DHW, yellow for 1-4, orange for 4-8, and red for 8+.
Task 2 - Identify risk zones: Which reef regions in your dataset have Watch-level or higher risk? Name specific locations or reef systems. Based on the map, which reefs are most stressed?
Task 3 - Interpret the data: Look at the historical context. Has this location experienced high DHW in previous years? If your dataset includes a time series (multiple weeks or months), show how DHW changed over time.
Task 4 - Predict outcomes: In locations now at 4-8 DHW, predict what field surveys would find: high bleaching incidence, moderate bleaching, or minimal bleaching? Explain your reasoning based on what you've learned about thermal stress thresholds.
1. What does the satellite sensor on MODIS measure to determine sea surface temperature?
2. Why does NOAA compare current temperatures to historical maximum monthly averages rather than to absolute temperature values?
3. If a reef location has a long-term maximum monthly average of 28°C and experiences a week at 29°C, how many DHW does that contribute?
4. What is the approximate DHW threshold above which severe coral bleaching is expected?
5. Explain why DHW is cumulative (weeks add together) but also "ages out" (old weeks get replaced with recent weeks).
You now understand how satellites measure temperature and how scientists translate that data into thermal stress metrics. In Level 3, you'll learn how artificial intelligence can automatically detect bleached reefs in satellite imagery, enabling faster monitoring across thousands of reef sites simultaneously.