The Northwestern Hawaiian Islands, also called the Leeward Hawaiian Islands, are a group of mostly uninhabited islands and atolls located northwest of Kauai and Niʻihau in the Hawaiian island chain. Politically, these islands are part of Honolulu County in the U.S. state of Hawaii, except for Midway Atoll. Midway Atoll is a separate area not included in maps of Hawaii and is classified as one of the United States Minor Outlying Islands. The United States Census Bureau refers to this area, excluding Midway Atoll, as Census Tract 114.98 of Honolulu County. The total land area of these islands is 3.1075 square miles (8.048 km²). Except for Nīhoa, all the islands are located north of the Tropic of Cancer, making them the only islands in Hawaii outside the tropics.

Most of the islands have no permanent residents, with the exception of Midway Atoll, which has a small, rotating group of U.S. Fish and Wildlife Service workers and other staff. Kure Atoll and Tern Island have temporary workers who visit seasonally. Some islands were visited by Ancient Hawaiians, and Nīhoa shows signs of long-term human habitation.

The Northwestern Hawaiian Islands are part of the Papahānaumokuākea Marine National Monument, a large area protected for marine life. They are home to endangered animals such as the Hawaiian monk seal and green sea turtle. The islands also hold cultural significance for Native Hawaiians. Geologically, they are the oldest in the Hawaiian-Emperor seamount chain, shaped by volcanic activity and erosion over millions of years.

The Northwestern or Leeward Hawaiian Islands include

The islands

• Nīhoa is the youngest and tallest island in the NWHI, with 900-foot (270 m) vertical cliffs. It is the southwestern part of a former volcanic cone. Ancient Hawaiians may have lived here for a long time.
• Necker Island is hook-shaped and 270 feet (82 m) tall at its highest point. It has no plants and was used by Ancient Hawaiians for religious purposes, but not for long-term living.
• French Frigate Shoals is an atoll, the largest coral reef area in Hawaii, covering 200 square miles (520 km²). It includes many small islands, one of which, Tern Island, has an airport and human homes.
• Gardner Pinnacles consists of two small basalt peaks, the last rocky island in Hawaii. Though tiny, the surrounding reef is large and has many different species.
• Maro Reef is a very fertile reef system, often called a "coral garden."
• Laysan is a 913-acre (3.69 km²) low, sandy island with a natural lake inside, one of only five such lakes in Hawaii. It has possibly the most diverse ecosystem in the NWHI and is home to about two million seabirds of seventeen species.
• Lisianski Island, 400 acres (1.6 km²), is similar to Laysan but lacks a lake. While slightly less biodiverse, its surrounding reef is very fertile.
• Pearl and Hermes Atoll is an atoll similar to French Frigate Shoals but has much less dry land. Because of this, it was mostly ignored by guano miners and feather hunters.
• Midway Atoll is the most well-known and largest island in the NWHI. The Battle of Midway took place here and nearby. The island is permanently inhabited by people who live there because of their government service, not an indigenous population.
• Circular Kure Atoll includes Green Island, a 236-acre (0.96 km²) island that once had a LORAN station and runway, but these are no longer in use. Kure is one of the less biodiverse islands in the NWHI.

Other islands or reefs were once mapped as part of this chain but are now considered either phantom islands or incorrect identifications of existing islands. These reefs appeared on maps as late as 1934:
• Two Brothers Reef, the site of a shipwreck in 1823, was originally placed west of French Frigate Shoals but later found to be the same location.
• "Krusenstern Reef," "Krusenstern Rock," or "Krusenstern Island" was a phantom reef located at 22°15′N 175°37′W, south of Lisianski Island. An investigation in 1923 confirmed it did not exist.

As late as 1960, a German globe showed islands west of Kure Atoll that had long been proven nonexistent:
• Byers's Island
• Morrell's Island

Geology

The Northwestern Hawaiian Islands were created about 7 to 30 million years ago by shield volcanoes over the same volcanic hotspot that formed the Emperor Seamounts to the north and the Main Hawaiian Islands to the south. As the Pacific Plate moved north and later northwest over the hotspot, volcanic eruptions formed islands in a straight line. Over time, these islands eroded and sank, changing from high islands near the south, similar to the Main Hawaiian Islands, to atolls or seamounts north of Darwin Point. Each island in the Northwestern Hawaiian Islands is at a different stage of erosion. Nīhoa, Necker, and Gardner Pinnacles are rocky, basalt islands that have not eroded enough to form atolls or have no large coral reefs. Laysan and Lisianski are low, sandy islands that have eroded for a longer time. French Frigate Shoals, Pearl and Hermes, Midway, and Kure are atolls.

North of Darwin Point, coral reefs grow more slowly than the islands sink. As the Pacific Plate moves northwest, islands cross this line and become seamounts. Kure Atoll is partly located at Darwin Point and will eventually sink beneath the ocean when its coral reef cannot keep up with the sinking. This will happen to all Hawaiian islands over time.

Biodiversity and endemism

The Hawaiian Islands are about 2,500 miles (4,000 km) from North America and 3,800 miles (6,100 km) from Asia. This distance has helped the islands develop many species that are found nowhere else in the world. Only animals that can fly or swim long distances, like birds or fish, could reach the islands. However, people who lived on the islands, such as Polynesians and Europeans, have changed the environment of the main Hawaiian islands by bringing in non-native species. In contrast, the ecosystems of the Northwestern Hawaiian Islands (NWHI) have remained mostly untouched. The coral reefs in Papahānaumokuākea are home to more than 7,000 marine species. Of these, over 1,700 species are found only in the Hawaiian Islands. Because of this, the area is sometimes called "America's Galápagos."

The Leeward Islands have not faced as much extinction as the main islands, but they have still suffered harm. From the late 1800s to the early 1900s, people who fished, mined guano, and hunted feathers killed many birds and sea animals in the NWHI. Rabbits were brought to Laysan and Lisianski, where they grew quickly and destroyed much of the plants, leading to the loss of several species. However, most of the damage has been fixed, and the islands have been restored to a state similar to how they were before humans arrived.

The NWHI has been recognized as an Important Bird Area (IBA) by BirdLife International because of its many seabirds and birds that live only on land. The seabird colonies in this area are among the largest groups of tropical seabirds in the world, with more than 14 million birds of 21 different species.

Some species that are found only in the NWHI include the Nīhoa and Laysan finch, the Laysan duck (the rarest native waterfowl in the United States), and the Nīhoa fan palm. Other important species are the Laysan albatross, the highly endangered Hawaiian monk seal, and the green sea turtle. The only native trapdoor spiders in the Hawaiian islands (Nihoa spp.), which were recently discovered, are also found here. Many of these unique species are at risk of extinction because a single major disaster could destroy all the plants on the small islands. Additionally, 70% of all coral reefs in the United States are located in the NWHI.

Climate

The Northwestern Hawaiian Islands have a dry-summer tropical savannah climate, as described in the Köppen climate classification system. In the driest month, which is May, these islands receive less than 60 mm (2.4 in) of rainfall. They also receive less than 100 minus (total annual precipitation in mm divided by 25) mm of rainfall. These amounts are the limits that define a tropical savanna climate compared to a tropical monsoon or rainforest climate.

Because these islands are small, rainfall and temperature are more uniform across and within them than on the larger Windward Islands (the main islands) of Hawaii. Winter months bring much larger ocean waves. On average, these islands receive about 500 to 750 millimeters (20 to 30 inches) of rainfall each year.

Exploration

Archaeological evidence shows ancient Hawaiians visited but did not live on Mokumanamana (Necker) and French Frigate Shoals. These islands were empty when Europeans arrived in the 18th century. Agricultural terraces show that Hawaiians lived on Nīhoa for a long time. Mokumanamana has no vegetation and is not suitable for farming. Archaeological studies show early Hawaiians only visited and used the island for religious purposes.

The first of the Leeward Isles discovered by Europeans was Nīhoa. James Colnett found it in 1786, though William Douglas is often credited. Later that year, La Pérouse discovered Necker and named it after Jacques Necker, the French Minister of Finance. La Pérouse then discovered French Frigate Shoals. Midway Atoll was the last of the NWHI to be found, discovered by N.C. Middlebrooks in 1859. In 1925, the Tanager Expedition traveled to many of the NWHI. The islands were mapped, new species were found and described, and archaeological sites on Nīhoa and Necker were discovered.

Most of the islands have several names: one in English and one or more in Hawaiian (as shown in parentheses above). Most Hawaiian names used instead of English names were created in modern times. The original names that ancient Hawaiians used for these islands before Western contact are found in oli (chants) and moʻolelo (stories).

National Monument

On June 15, 2006, American President George W. Bush made a public statement to create the Papahānaumokuākea Marine National Monument using the Antiquities Act of 1906. This monument includes the islands and the surrounding ocean, making it the world’s largest marine wildlife reserve. President Theodore Roosevelt had protected the Northwestern Hawaiian Islands as a bird sanctuary in 1909, and President Bill Clinton had designated the area as an "ecosystem reserve" in 2000. However, changing the area to a national monument status gave it stronger protection. At that time, 139,000 square miles (360,000 km²) of ocean were set aside for protection, an area about the size of the U.S. state of California.

In August 2016, President Barack Obama expanded the monument’s area by about four times. After this expansion, the monument became the world’s largest marine protected area.

Access to the monument is controlled through a special permission system managed by NOAA, FWS, and the state of Hawaii. Anyone visiting the islands must follow strict rules to stop unwanted species from entering and harming the ecosystem. All clothing must be purchased new and kept wrapped until arriving. Soft items, like camera straps or blankets, must also be new, while hard items, such as cameras or binoculars, must be cleaned thoroughly. Every item must then be frozen for 48 hours. New equipment must be prepared for each island visited to prevent the spread of species between islands. French Frigate Shoals and Midway Atoll are not required to follow these rules because human activity has already changed their environments too much to worry about new species.