Nine Hours in Twenty-Four: The Rule Book That Makes the Keeling Curve

The Traveler's Panel · checked 22 August 2026

Changing information. Prices, permits, seasons and road access change, sometimes at short notice. Everything here is a starting point for your own confirmation — not a quote, and not a guarantee. Verify with the operator and the relevant official body before you book.

Getting there — Hilo → Daniel K. Inouye Highway (Route 200) → Mauna Loa Observatory Road — closed to the public

1. You cannot go. This is not a permit puzzle to be solved with paperwork and patience: NOAA's own status notice states that there is no site access for the general public to the Mauna Loa Observatory and that no public opening date has been established. Before the eruption the laboratory ran occasional tours; the page describing them is currently unreachable, and there is nothing to book.
2. The physical route, for orientation only. The observatory sits on the north flank of Mauna Loa at 19.5362°N, 155.5763°W. The approach is from the Daniel K. Inouye Highway, the road across the saddle between Mauna Loa and Mauna Kea, which Hawai'i's Department of Transportation describes as a high-speed facility with sections posted at 60 mph and with enforced no-parking zones between mile markers 16 and 31. The observatory spur climbs roughly seventeen miles from there to 11,135 feet.
3. That spur is the thing the 2022 eruption ate. On the night of 28 November 2022 a lava flow crossed the Mauna Loa Weather Observatory Road at about 8pm, burying roughly a mile of it under deposits up to thirty feet deep and taking out sixty power poles. For more than a year NOAA staff reached their own observatory once a week by helicopter. Construction of a temporary 1.2-mile access road across the flows was completed on 26 March 2026.
4. The road being finished is not the same as the site being open, and NOAA has been careful to say so. It is a working federal atmospheric baseline station running on solar panels and batteries, with mission-critical instruments being brought back online in sequence. There is no visitor infrastructure and nothing at the top for a member of the public to do.
5. If you want the substance of this place rather than the address of it, the honest route is the data. NOAA publishes the Mauna Loa record at monthly, weekly and daily resolution, updated continuously, alongside the documentation of exactly which hours were used to build it. Reading the selection-criteria page next to the graph takes about twenty minutes and is the closest thing to a visit that is currently available to anybody outside the laboratory.

Indicative costs — verify before booking

What to pack

• Nothing. There is no lawful way for a visitor to be at this site, and the responsible packing list for a working atmospheric baseline observatory is an empty one.
• For the saddle itself, if you are crossing it lawfully on the highway: the Daniel K. Inouye Highway is a fast road across open lava at over 6,000 feet, with enforced no-parking zones and no shoulder to improvise on. Fuel and water before you start; there is nothing between the two coasts.
• For Maunakea, if you are going to the Visitor Information Station at 9,200 feet: warm layers and time to acclimatise. Four-wheel drive is required above the station, and the University of Hawai'i advises that pregnant women, people in poor health and children under 13 should not go higher.
• A copy of NOAA's selection-criteria page, read before you look at the Keeling Curve again. It is the single most useful object connected with this place.

Where to stay

Nobody stays at the observatory and nobody should try. The practical base for anything on this side of the island is Hilo, 19.72991°N 155.09073°W, a city of about 43,263 people at 18 metres above sea level with the island's main airport four kilometres away — roughly fifty miles and eleven thousand vertical feet from the instrument this dossier is about. Kailua-Kona, on the dry western coast at 19.64016°N 155.99912°W, is the alternative and the busier one. Between them runs the Daniel K. Inouye Highway across the saddle, which is the only road that puts a traveller anywhere near either summit; the Mauna Kea Visitor Information Station at 9,200 feet is the highest point on this island a visitor can lawfully reach without special arrangements, and it is on the other mountain. In August 2026 the wider picture is worse than usual: Hurricane Lala destroyed roads and bridges in the Kaʻū District, left Hawai'i Volcanoes National Park without power, phones or internet, and a further storm closed the park again on 21 August. Check everything twice before booking anything.

Safety & responsible travel

• Do not repeat the nine-hours figure as though it were a revelation of concealment. NOAA publishes it, publishes every flagged hour alongside it, and states that no data are thrown away. The selection removes local contamination — island vegetation drawing CO₂ down by day, combustion below, and volcanic CO₂ from Mauna Loa's own crater — and two independent programmes applying different rules agree to 0.04 ppm.
• Do not treat Maunakea as a convenient outbuilding of the CO₂ record. It is a place of deep cultural and religious significance to Native Hawaiians, its governance is the subject of live state legislation and live litigation, and the observatories there operate under an access regime that is genuinely contested. Anyone writing about the seven months the curve spent up there owes that context.
• Do not drive up the observatory spur to look. It is closed, it crosses a 2022 lava flow on a temporary alignment, and the people using it are servicing instruments on battery power.
• Do not park on the Daniel K. Inouye Highway to photograph the mountains. Hawai'i DOT enforced no-parking zones across a fifteen-mile stretch during the eruption for exactly this reason.
• Check the National Park Service before assuming anything about Hawai'i Volcanoes National Park is open. Hurricane Lala left the park without power, phones or internet in August 2026, and it closed again on 21 August ahead of a further storm.

Nearby, and quieter

The other mountain is the story. Maunakea, 4,207 metres and about 21 miles north, hosted the world's most famous CO₂ record for seven months while Mauna Loa's road lay under lava, and it is a summit of profound cultural and genealogical significance to Native Hawaiians whose management passed in 2022 to a new eleven-member Stewardship and Oversight Authority under Act 255, with the Office of Hawaiian Affairs suing in January 2024 to have the Act repealed. The Mauna Kea Ice Age Natural Area Reserve, at 19.78689°N 155.4698°W and 3,612 metres, protects the highest glacial landscape in the Pacific. South of the saddle, Hawai'i Volcanoes National Park contains the high Northeast Rift Zone fissures that fed the 2022 flows, though the park is currently working through a two-year construction programme at the Kīlauea summit and, in August 2026, storm damage on top of it. And Mauna Loa itself — 4,169 metres, 34 eruptions since 1843, alert level green as of 7 August 2026 — is the only thing on this list that has ever succeeded in stopping the Keeling Curve.

Getting there, in order

• 1Hilo
• ↓
• 2Daniel K. Inouye Highway (Route 200)
• ↓
• 3Mauna Loa Observatory Road — closed to the public

Each leg is a separate booking and a separate chance to be weathered out. Build spare days into the chain, not onto the end of it.

Where it is

Mauna Loa Baseline Observatory, north flank of Mauna Loa, Hawai'i · 19.5362°N 155.5763°W · Open in Google Maps · OpenStreetMap

Key locations

• Mauna Loa Baseline Observatory — NOAA's own site metadata; 3,397 m; no public access and no opening date
  19.5362°N 155.5763°W
• Mauna Loa summit — GeoNames; USGS gives 4,169 m and 34 eruptions since 1843
  19.4722°N 155.5922°W
• Mauna Kea summit — GeoNames; USGS gives 4,207 m — the curve's temporary home, Dec 2022 to Jul 2023
  19.8236°N 155.4708°W
• Halepōhaku, Mauna Kea State Park — Visitor Information Station; 2,804 m per UH Hilo, 2,884 m per GeoNames
  19.7639°N 155.4583°W
• Mauna Kea Ice Age Natural Area Reserve — GeoNames; 3,612 m
  19.7869°N 155.4698°W
• Hilo — GeoNames; population 43,263 at 18 m — the practical base
  19.7299°N 155.0907°W
• Hilo International Airport — GeoNames; the arrival point for the windward side
  19.7161°N 155.0561°W
• Kailua-Kona — GeoNames; population 11,975 — the leeward alternative
  19.6402°N 155.9991°W

Coordinates are for orientation and are accurate to roughly the width of the feature named, not to a doorway. Several of these points are inside protected areas or need a permit, a guide or a boat — read the access notes above before you plan a route to any of them.

Sources & further reading — checked 22 August 2026

1. NOAA Global Monitoring Laboratory — 'How we measure background CO₂ levels on Mauna Loa'. The full selection rule book: the V flag for hourly standard deviation of five-minute averages above 0.30 ppm, the U flag for 11am–7pm local time (21–5 UTC), the D flag for hour-to-hour change above 0.25 ppm, the S and N isolation flags, the statement 'No data are thrown away', 'On average over the entire record there are 9.0 retained hours per day with background CO₂ mole fractions', the 2014 breakdown (3,323 unflagged hours, 37.9%; 2,680 U, 30.6%; 1,063 D, 12.1%), the two intakes on a 38 m tower, the cold-trap drying, the CRDS description, the April 2019 changeover from infrared absorption, and 'the observatory is surrounded by many miles of bare lava, without any vegetation or soil'
2. NOAA Global Monitoring Laboratory — 'Carbon Cycle Greenhouse Gases: CO₂ measurements (NDIR)'. The pre-2019 infrared analyser and its hourly cycle (minutes 0–24 line 1, 25–44 line 2, 45–59 the three reference gases W3, W2, W1); the same five selection flags; and the comparison with Scripps — 'the average difference during 1974-2004 between the Scripps and our monthly means is 0.04 ppm, and that the standard deviation of the annual mean differences is 0.12 ppm', plus the statement that Scripps 'have always maintained their own manometric calibration scale, they use a different analyzer system, and their data selection techniques for selecting background air are different from, and independent of, our methods'
3. NOAA Global Monitoring Laboratory — 'Trends in Atmospheric Carbon Dioxide: Mauna Loa'. July 2026 monthly mean of 429.12 ppm against 427.87 ppm in July 2025; the statement that measurements 'were suspended as of Nov. 29, 2022'; that 'Observations from December 2022 to July 4, 2023 are from a site at the Maunakea Observatories, approximately 21 miles north of the Mauna Loa Observatory'; resumption in July 2023; Scripps measurements from March 1958 and NOAA's from May 1974 run in parallel; and the preliminary status of the last year of data
4. NOAA Global Monitoring Laboratory — Mauna Loa weekly CO₂. The week beginning 16 August 2026 at 426.98 ppm, against 425.60 ppm a year earlier and 402.11 ppm ten years earlier, last updated 23 August 2026; average day-to-day variability of 0.38 ppm across the record; and the same eruption and Maunakea relocation notice
5. NOAA Global Monitoring Laboratory — Mauna Loa Observatory site page and status notice. The status notice re-checked on the day of writing: the 2022 eruption cutting road access; 'Construction on the temporary Mauna Loa Access road was completed on 26 March, 2026'; no public site access and no established public opening date; and 'It is still unclear when utility infrastructure will be replaced and power re-established to the site.' Also the elevation of 3,397 m / 11,135 ft and position at 19.5362°N, 155.5763°W
6. NOAA Global Monitoring Laboratory — Mauna Loa Observatory, About Us. The observatory constructed in 1956 and continuously in operation since; its status as an atmospheric baseline station of the Global Monitoring Laboratory; the elevation of 3,397 m; and the description of the site protruding through the marine temperature inversion layer which separates polluted lower air from the cleaner free troposphere
7. NOAA Global Monitoring Laboratory — measurement site record for MLO. Site metadata: latitude 19.5362°N, longitude 155.5763°W, elevation 3,397.00 m above sea level, site code MLO, local standard time plus 10 hours equals UTC, established 1957, and the ten programmes running there including carbon cycle in situ observations from 1974
8. NOAA Global Monitoring Laboratory — Mauna Loa Observatory programmes. The seven measurement programmes at the site — greenhouse gases and carbon cycle, ozone and water vapour, halocarbons and atmospheric trace species, aerosols and air quality, lidar, solar radiation, and NDACC — and the note that Scripps's critical CO₂ records were restored
9. NOAA Global Monitoring Laboratory — Scripps CO₂ cooperative programme at Mauna Loa. NOAA's record of the Scripps programme at the site: date started January 1958, and continuous CO₂ concentrations recorded with a non-dispersive infrared instrument drawing from three possible inlets
10. NOAA Global Monitoring Laboratory — Central Calibration Laboratory. That NOAA GML is the WMO Global Atmosphere Watch Central Calibration Laboratory for CO₂, CH₄, N₂O, SF₆ and CO, and that scales are named for the year in which they are adopted
11. NOAA Global Monitoring Laboratory — the WMO CO₂ calibration scale. WMO-CO2-X2019 based on 19 primary standards covering a nominal range of 250–800 ppm; value assignment from manometric determinations repeated approximately every two to three years, the latest included episode being 2015–2017; retroactive application to all NOAA calibrations; and the hierarchy of primary, secondary and tertiary standards
12. NOAA Global Monitoring Laboratory — Mauna Loa news listing. The dated sequence of announcements: 19 December 2022 on the University of Hawai'i partnership at the Mauna Kea Observatories, 6 February 2023 on the planned renovation once road access is restored, 6 June 2024 on accelerating CO₂ accumulation, and the 26 March 2026 road-completion notice with no public opening date
13. NOAA Research — 'One year after the eruption, Mauna Loa Observatory has resumed key science activities' (13 December 2023). Approximately one mile of access road buried under lava up to 30 feet deep and 60 power poles destroyed; staff visiting once a week by helicopter; limited power restored to four key buildings by augmenting solar generation and adding batteries; roughly 33 per cent of measurements restored; and Vanda Grubišić's statement that 'the interruption in the key long-term records was only 10 days'
14. NOAA — 'University of Hawaii, NOAA to gather climate change data following Mauna Loa eruption'. That the replacement system was installed on 8 December 2022 at the Maunakea Observatories, intended to provide records of global CO₂ similar to those gathered for more than six decades at Mauna Loa, with Steve Thur quoted on the value of the Hawai'i record
15. Scripps Institution of Oceanography — Scripps CO₂ Program. That the programme began in 1957 at La Jolla and the South Pole and in 1958 at Mauna Loa; that the primary in situ Mauna Loa record is based on measurements made with an analyser at the site while other station records are based on flask samples returned to La Jolla; and that isotopic measurements began in 1978
16. Scripps CO₂ Program — Primary Mauna Loa CO₂ record. The record running 1958 to present at 19.5°N, 155.6°W, 3,397 m, published as monthly means in CSV, with Ralph Keeling named as the contact for methodological questions
17. The Keeling Curve (Scripps) — 'Historic Keeling Curve Observation Interrupted by Volcanic Eruption' (29 November 2022). The eruption at 1:30am PST on 28 November 2022 and the loss of power to the CO₂ equipment at 6:30pm the same day; the 1984 precedent of a gap from 26 March to 29 April closed by bringing a generator to the observatory; and Ralph Keeling's assessment, 'It's a big eruption and it's in a bad place'
18. The Keeling Curve (Scripps) — 'Update on CO₂ readings commencing at Maunakea' (10 January 2023). That the Scripps analyser at Maunakea has been operational since 14 December 2022; that it is a nearly identical instrument; that results are processed using the same algorithms used at Mauna Loa; and that the plan was to continue Maunakea measurements for at least a full year after Mauna Loa resumed, to allow close comparison
19. The Keeling Curve (Scripps) — 'The History of the Keeling Curve'. The first Mauna Loa reading of 313 ppm in March 1958; the statement that the Scripps programme 'was actually shut down for a period in 1964 following congressionally mandated budget cuts'; the funding threats through the 1970s and 1980s; and the description of redundancy between the Scripps and NOAA programmes as essential to a robust long-term measurement programme
20. Tellus (Series A) — C. D. Keeling, 'The Concentration and Isotopic Abundances of Carbon Dioxide in the Atmosphere', vol. 12 no. 2, pp. 200–203 (1960), DOI 10.3402/tellusa.v12i2.9366. The founding paper: three gas analysers with strip chart recorders at stations in Antarctica, Hawai'i and California; variability at Mauna Loa in approximately half the records attributed to nearby volcanic vents, island combustion and upslope air; the selection statement that 'The values reported here are averages of data for periods of downslope winds or strong lateral winds when the concentration remained nearly constant for several hours or more'; and a northern Pacific increase of between 0.5 and 1.2 ppm per year
21. Atmospheric Measurement Techniques — Hall and others, 'Revision of the World Meteorological Organization Global Atmosphere Watch (WMO/GAW) CO₂ calibration scale', 14, 3015–3032 (2021). The definition of WMO-CO2-X2019; the new scale being 0.18 ppm greater than its predecessor at 400 ppm, a change of 0.045%, called significant for atmospheric monitoring; 19 primary standards including four new ones extending to 800 ppm; the expanded uncertainty of 0.17 ppm (0.043%) at 400 ppm; and the network compatibility goals of 0.1 ppm in the Northern Hemisphere and 0.05 ppm in the Southern
22. World Meteorological Organization — Greenhouse Gas Bulletin No. 21 (October 2025). Global mean CO₂ of 423.9 ± 0.2 ppm for 2024, an increase of 3.5 ppm on 2023 described as the largest one-year rise since modern measurements began in 1957, 152% of the 278.3 ppm pre-industrial level, from 179 contributing stations, with Scripps Mauna Loa and South Pole measurements used for the 1960–1979 portion of the long record
23. American Chemical Society — National Historic Chemical Landmark, 'The Keeling Curve'. The landmark dedications on 30 April 2015 at Mauna Loa Observatory and 12 June 2015 at Scripps; Keeling's manometer controlled for temperature, pressure and volume to a precision of 0.1%; the Applied Physics Corporation infrared gas analysers; and 'The first reading from Mauna Loa, dated March 29, 1958, measured the atmospheric CO2 concentration at 313 ppm'
24. American Physical Society — 'March 1958: Charles Keeling begins long-term measurements of atmospheric CO₂ on Mauna Loa'. The 29 March 1958 first reading at 313 ppm; the use of infrared spectrometers giving continuous readouts; and the note that a small gap occurred in 1964 when instruments broke down after funding cuts, with no duration specified
25. US Geological Survey — 'November 27–December 10, 2022 Eruption of Mauna Loa'. Lava breaking to the surface at approximately 11:30pm HST on 27 November 2022; fissures at high elevation on the Northeast Rift Zone within Hawai'i Volcanoes National Park; flows extending 12 miles (19 km) north-northeast and stopping within 1.7 miles (2.8 km) of the Daniel K. Inouye Highway; and the flow crossing the Mauna Loa Weather Observatory Road at approximately 8pm on 28 November
26. US Geological Survey Hawaiian Volcano Observatory — 'New USGS Data Release: Mauna Loa 2022 eruption geospatial data'. The eruption continuing for twelve days, lava covering approximately 36 square kilometres (14 square miles), flows reaching 18 kilometres (11 miles) north and up to 3 kilometres (2 miles) south-west, and the released geodatabase, shapefiles, KMZs and thermal map rasters
27. US Geological Survey — 'March 25–April 15, 1984 Eruption of Mauna Loa'. The satellite infrared signal at 1:25am on 25 March 1984, the migration of activity to the north-east rift zone with vents at 2,850 m, the flow advancing 29 km by 29 March to within about 6.5 km (4 miles) of the outskirts of Hilo, and the eruption ending on 15 April
28. US Geological Survey — Mauna Loa volcano summary. Summit elevation of 4,169 m (13,679 ft); 34 eruptions since the first well-documented historical eruption in 1843; the most recent beginning 27 November 2022; and the current aviation and volcano alert level of green/NORMAL as of 7 August 2026
29. US Geological Survey — Mauna Kea volcano summary. Summit elevation of 4,207 m (13,803 ft) at approximately 19.82°N, 155.47°W; most recent eruption between 6,000 and 4,000 years ago; a moderate threat rating; and the description of the volcano as a shield built on the flank of Kohala's eastern rift zone
30. US Geological Survey Hawaiian Volcano Observatory — 'Volcano Watch: Mauna Loa Observatory, Keeling Curve recognized as landmark'. The observatory placed at 3,350 m (11,000 ft) on Mauna Loa's north flank; the National Historic Chemical Landmark designation of 30 April 2015; and the statement that MLO scientists 'are careful to exclude these data from the background CO₂ record' when volcanic CO₂ reaches the site, having also used the excluded records to estimate Mauna Loa's own emissions
31. State of Hawai'i — Act 255, Session Laws of Hawai'i 2022 (Mauna Kea Stewardship and Oversight Authority), as published by the Department of Land and Natural Resources. The legislative finding that 'Mauna Kea serves as an important cultural and genealogical site to the people of Hawaii, particularly to Native Hawaiians'; the eleven-member Authority including a lineal descendant of a practitioner and a recognised practitioner of Native Hawaiian traditional and customary practices; the five-year transition from 1 July 2023 with full transfer from the University of Hawai'i on 1 July 2028; the bar on new or renewed leases during transition; and the duty to protect traditional and customary rights and 'not unduly burden individuals exercising these rights'
32. Office of Hawaiian Affairs — 'OHA lawsuit seeks repeal of the Mauna Kea Stewardship and Oversight Authority'. That the Authority began meeting in June 2023 to manage Mauna Kea lands held in the ceded lands trust; that OHA has no representative on it; that OHA filed suit on 17 January 2024 challenging the Act's legality and constitutionality; and Board Chair Carmen 'Hulu' Lindsey's statement that 'Mauna Kea represents a sacred space for Native Hawaiians'
33. University of Hawai'i at Hilo, Center for Maunakea Stewardship — Visitor Information Station. The station at 9,200 ft (2,804 m) open 9am to 9pm daily; the summit road above it unpaved, rough and steep with four-wheel drive required; and the warning that pregnant women, people in poor health and children under 13 should not go above the station because of altitude
34. National Park Service, Hawai'i Volcanoes National Park — 'Most of Hawaiʻi Volcanoes National Park Remains Closed and Without Power Due to Hurricane Lala Impacts' (17 August 2026). Hurricane Lala causing widespread island-wide devastation, particularly in the Kaʻū District where roads and bridges were destroyed by flooding; campgrounds, backcountry, Volcano House and Kilauea Military Camp closed; 'The park still has no power, phones, or internet'; and downed trees cleared from Mauna Loa Road among others
35. National Park Service, Hawai'i Volcanoes National Park — 'Hawaiʻi Volcanoes National Park Will Close Friday Ahead of New Storm System' (20 August 2026). The park and its Kahuku Unit closing by 5pm on Friday 21 August 2026 ahead of a storm system forecast to bring torrential rainfall and high winds to Hawai'i Island
36. Hawai'i Department of Transportation — 'Reminder: Do not park on DKI (Saddle Road) to observe Mauna Loa eruption'. Enforcement of no-parking zones between mile markers 16 and 31 on the Daniel K. Inouye Highway during the eruption, and the description of DKI as a high-speed facility with sections posted at 60 mph
37. GeoNames gazetteer — Hawai'i Island features. The coordinates used in this dossier's panel: Hilo at 19.72991°N 155.09073°W, population 43,263; Hilo International Airport at 19.71607°N 155.05606°W; Kailua-Kona at 19.64016°N 155.99912°W; Mauna Loa at 19.47217°N 155.59221°W; Mauna Kea at 19.82361°N 155.47083°W; and the Mauna Kea Ice Age Natural Area Reserve at 19.78689°N 155.4698°W
38. GeoNames gazetteer — Mauna Kea State Park, Halepōhaku area. The Halepōhaku area of Mauna Kea State Park at 19.76389°N, 155.45833°W, given an elevation of 2,884 m — a figure that does not match the 2,804 m the University of Hawai'i gives for the Visitor Information Station at the same place
39. Big Island Video News — 'Mauna Loa Observatory Road Restored, 3 Years After Lava Flow' (20 May 2026). Cited only for the announcement and the interview: NOAA Global Monitoring Laboratory director Vanda Grubišić dating the reopening of the road to MLO to 26 March 2026, the temporary route running about 1.2 miles across the 2022 flows, no timeline yet set for restoring power, and limited solar and battery systems installed in mid-2023 supporting 68% of measurement programmes

Official and scientific bodies are the authority for safety, regulatory and scientific claims. Commercial operators are used only for practical detail such as tour length, meeting points and indicative pricing — never for a safety or scientific statement.