The Thermal Equator on Earth and Mars

Christopher P. McKay Space Science Division, NASA Ames Research Center, Moffett Field, California

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Mateo N. Cintron Space Science Division, NASA Ames Research Center, Moffett Field, California

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Abstract

The thermal equator (also known as the heat equator) is the circumplanetary set of points that represent the highest mean annual temperature at each longitude. Recent high precision global datasets for Earth and Mars provide a basis for a detailed calculation of the thermal equator on these worlds. On Earth, the temperature values that comprise the thermal equator range from 25.85° to 34.75°C, with a mean of 27.75° ± 1.3°C, and extends in latitude as high as 20°N in Mexico and 29.3°N in the Indian subcontinent. The maximum southern extent is 20°S in Australia. On Mars, lacking oceans, the thermal equator takes a simpler track and is roughly parallel to the equator, and displaced 5°–10°S. However, there is a region of longitude on Mars where the thermal equator becomes bimodal with a northern branch centered at 10°N and a southern branch centered at 20°S.

For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Christopher P. McKay, chris.mckay@nasa.gov

Abstract

The thermal equator (also known as the heat equator) is the circumplanetary set of points that represent the highest mean annual temperature at each longitude. Recent high precision global datasets for Earth and Mars provide a basis for a detailed calculation of the thermal equator on these worlds. On Earth, the temperature values that comprise the thermal equator range from 25.85° to 34.75°C, with a mean of 27.75° ± 1.3°C, and extends in latitude as high as 20°N in Mexico and 29.3°N in the Indian subcontinent. The maximum southern extent is 20°S in Australia. On Mars, lacking oceans, the thermal equator takes a simpler track and is roughly parallel to the equator, and displaced 5°–10°S. However, there is a region of longitude on Mars where the thermal equator becomes bimodal with a northern branch centered at 10°N and a southern branch centered at 20°S.

For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Christopher P. McKay, chris.mckay@nasa.gov

1. Introduction

The meteorological boundaries between the Northern and Southern Hemispheres are generally not aligned with the geographic equator. Most prominently, the intertropical convergence zone (ITCZ) is a band of clouds that circles Earth resulting from upwelling in the global atmospheric circulation driven by solar heating (e.g., Waliser and Gautier 1993). The ITCZ moves north and south about the geographical equator with the seasons, and theoretically, the ITCZ should follow the subsolar point. Operationally, the location of the ITCZ is defined by a maximum in precipitation in the tropical regions (e.g., Utida et al. 2019).

The meteorological equator refers to the zone of low pressure near the surface that sits between the circulation patterns of the Northern and Southern Hemispheres. This low-pressure trough contains the ITCZ and moves during the seasons, generally toward the summer hemisphere. The mean location of the meteorological equator is about 5°–6°N, and this latitude itself is sometimes referred to as the meteorological equator (e.g., Ramage and Hori 1981).

Kang et al. (2008) defined the energy flux equator as the latitude where the atmospheric meridional energy transport is zero—no energy transport between hemispheres. It is related to the ITCZ and similarly moves with the seasons (Kang et al. 2008, 2009; Boos and Korty 2016; Adam et al. 2016).

The ITCZ, the meteorological equator, and the energy flux equator are all atmospheric phenomena. In contrast, the thermal equator refers to surface temperatures. The thermal equator is defined in the American Meteorological Society Glossary of Meteorology as “The line that circumscribes the earth and connects all points of highest mean annual temperature for their longitudes.” This is sometimes referred to as the heat equator. The term thermal equator is also used, albeit less often, as equivalent to the ITCZ (e.g., Putnam et al. 2012; Broecker and Putnam 2013; Putnam and Broecker 2017; Hatchett 2018). For example, Hatchett (2018) defines “Earth’s thermal equator—the annually migrating belt of hottest surface temperatures—and thereby the position of tropical rain belts and mid-latitude storm tracks.” This is not the thermal equator we are looking for. The term thermal equator is also sometimes used to refer to the constant latitude (about 10°N) that has the highest mean annual temperature of any latitude. Here, we consider only the primary definition as stated in the American Meteorological Society (AMS) glossary.

The thermal equator, properly so-called, is of interest in ecological studies which depend on mean annual temperatures. For example, in the seasonal cues used for plant flowering (Wright 2016) and in the determination of the elevation of tropical treelines (Körner and Paulsen 2004). Thus, a precise map of the thermal equator may be of use in the analysis of ecological data in the tropics. On Mars, the thermal equator may be of interest in models of subsurface ground ice and in terms of locating research bases. In these applications, the persistence of ice or liquid water in the subsurface depends on the mean annual temperature. In this paper, we use recent high-resolution datasets for surface temperature on Earth (Karger et al. 2017) and Mars (Kieffer 2013) for an accurate computation of the thermal equator on both worlds.

To our knowledge, a detailed analysis of the thermal equator on either Earth or Mars has not been published; hence, we felt it would be useful to prepare such an analysis and to archive a digital version at the resolution of the existing datasets.

2. The thermal equator on Earth

High resolution climate data for Earth have been collated and combined into standard products as documented by Karger et al. (2017). One of these products is CHELSA_bio1_1981–2010_V.2.1, a Geostationary Earth Orbit Tagged Image File Format (GeoTIFF) file listing the annual average temperature over Earth for the years 1981–2010. The dataset is the result of data integration that combines modeling results with ground and radiosonde observations as well as remote sensing data. The temperature resolution of the data we use is 0.1°C and the spatial resolution is 0.0083°. We use this dataset, shown in Fig. 1, to compute the thermal equator for Earth.

Fig. 1.
Fig. 1.

The mean annual temperature on Earth from 1981 to 2010. Data are from Karger et al. (2017) GeoTIFF file CHELSA_bio1_1981–2010_V.2.1. The temperature resolution of the data is 0.1°C and the spatial resolution is 0.0083°, extracted from the GeoTIFF format using the RasterIO programming package in Python.

Citation: Bulletin of the American Meteorological Society 105, 6; 10.1175/BAMS-D-23-0214.1

Figure 2 shows the thermal equator determined from Fig. 1. Figure 2a shows the temperature of the thermal equator as a function of longitude. Figure 2b shows the extent in latitude for which the mean annual temperature reaches the maximum value for that longitude in Fig. 2a. Figure 2c shows the location of the thermal equator as the average over the latitude extent defined in Fig. 2b.

Fig. 2.
Fig. 2.

The thermal equator. As a function of longitude: (a) the maximum annual average temperature, (b) the extent in latitude, and (c) average location. The base topographic map is from the NASA Shuttle Radar Topography Mission.

Citation: Bulletin of the American Meteorological Society 105, 6; 10.1175/BAMS-D-23-0214.1

There are several interesting features in the thermal equator shown in Fig. 2. Going from west to east, at about 120°W, the thermal equator moves sharply from 10°S to 10°N. This is because of the cold Humboldt Current that flows upward along the west coast of South America (e.g., Chavez et al. 2008). The minimum mean annual temperature in the thermal equator, 25.85°C, occurs in the Pacific Ocean off the coast of Central America. In Central America and the upper parts of South America, the thermal equator follows the land. In general, land has a higher mean annual temperature than the adjacent ocean. A sharp upward spike in latitude occurs at about 117°W where the thermal equator jumps from 10° to 36°N and then returns. This is due to the high mean annual temperature of Death Valley, California. The maximum value of the mean annual surface temperature in the thermal equator is 34.75°C and occurs in eastern Africa at ∼40°E. The average value of the mean annual temperature along the track of the thermal equator is 27.75° ± 1.3°C.

Setting aside Death Valley, the northernmost locations of the thermal equator are in Mexico (20°N) and at the western edge of the Indian subcontinent (29.3°N). The average latitude of the thermal equator in Fig. 2c is 5°N ± 11° (averaged for 43 200 longitudes).

The triangular shapes in the latitude width of the thermal equator seen in Fig. 2b, especially in the Pacific Ocean, are due to discretization effects in latitude, longitude, and temperature values. These occur when the temperature resolution in the data ΔT is such that ΔT > Δφ × ∂T/∂φ|TE, where Δφ is the step size in latitude and the derivative is evaluated at the thermal equator. This relationship implies that in general, many pixels at a given longitude will have the maximum temperature— the thermal equator at that longitude will be a continuous band in latitude, not a single pixel. And when in addition, ΔT ≫ Δλ × ∂T/∂λ|TE, where Δλ is the step size in longitude, and again the derivative is evaluated along the thermal equator, then it takes a large number of longitude steps before the temperature increases (or decreases) by one temperature step. The number of latitude pixels that share the maximum temperature decreases (or increases) as the longitude approaches the point where the temperature drops (or increases) by one step. Thus, each triangle is a step change in temperature of ΔT with the lower temperature at the point of the triangle. The effect is most visible in the oceans due to the much smaller and smoother temperature variations than on land, as is clear in Fig. 2a. Mars has no oceans, and the effect is not present. These artifacts of temperature resolution do not affect the average position of the thermal equator shown in Fig. 2c.

Over the longitude range from 110° to 145°E, the thermal equator appears to oscillate (Fig. 2c) over a large extent in latitude (Fig. 2b). This is examined in more detail in Fig. 3 which is a closeup for this region. In Fig. 3, all pixels with temperature equal to the maximum for that longitude are red. The northern limit of the zone of the thermal equator starts at 10°N and tracks upward through Malaysia and the Philippines, reaching a peak latitude of 20°N; the thermal equator then enlarges into a broad band over the ocean, above New Guinea, spanning from 10° to 20°N, and gradually shrinks to a thinner zone as it continues eastward over the ocean. The southern limit also starts at 10°N and then tracks down the island of Java in Indonesia before moving down to the coast of Australia at 115°E. The thermal equator follows the coast until reaching 145°E, where it jumps from 5°S to 5°N (into the northern branch).

Fig. 3.
Fig. 3.

Enlargement of the region near Australia but now with all pixels with mean annual surface temperatures equal to the maximum for that longitude marked red. In this area, the thermal equator is well represented as a band of latitude, ∼30° wide.

Citation: Bulletin of the American Meteorological Society 105, 6; 10.1175/BAMS-D-23-0214.1

In principle, there is no reason why the thermal equator must be single valued, or even a continuous zone, at every longitude. However, in this case, the data (as can been seen in Fig. 1) indicate that the surface of the ocean and the low elevation parts of the land have high mean annual surface temperatures throughout the zone defined by the northern and southern boundaries of the thermal equator over Australia. The temperature that brings the thermal equator firmly onto the coast of Australia between 125° and 135°E is only ∼1°C higher than the ocean temperature north of the coast. Essentially, over 115°–145°E, there is ∼30° zone in latitude that is at, or very close to, the maximum temperature. The thermal equator is properly represented here as a wide zone, not a line.

3. The thermal equator on Mars

For Mars, we use the global map of annual average surface temperature published by Kieffer (2013, Fig. 10). This dataset is the result of a thermal model calculation with parameters specified by spacecraft observations. The published results have a temperature resolution of ±1 K with 0.05° spatial resolution.

Figure 4 shows the thermal equator on Mars calculated in analogy with the thermal equator on Earth in Fig. 2c. Between 90°W and 135°E, the red line represents the average position of the thermal equator determined exactly as described for Earth in Fig. 2c. Outside this longitude range, the thermal equator on Mars bifurcates into a northern and a southern branch with points of maximum temperature on either side of a zone centered about 10°S that is significantly cooler than the maximum temperature. This is shown in Fig. 5 with the temperature data from Kieffer (2013). In these regions, there are two red lines, one for the average position of the thermal equator for those points north of the colder regions and one for the points south of the colder region. The mean annual temperature in both branches is ∼219 K, while between the branches, the mean annual temperature is considerably less, ∼210 K. The thermal equator here is bimodal and not simply wide. There is no counterpart to this on Earth.

Fig. 4.
Fig. 4.

The thermal equator on Mars showing the average position at each longitude of the locations with the maximum mean annual temperature at that longitude. Over the longitude range 135°E eastward to 90°W, the thermal equator has two separate branches.

Citation: Bulletin of the American Meteorological Society 105, 6; 10.1175/BAMS-D-23-0214.1

Fig. 5.
Fig. 5.

The bifurcation of the Mars thermal equator from Fig. 4 shown with the temperature data of Kieffer (2013, Fig. 10) in the background. Temperature scale: pink: 200 K, blue: 208 K, green: 216 K, and yellow: 222 K.

Citation: Bulletin of the American Meteorological Society 105, 6; 10.1175/BAMS-D-23-0214.1

Along the thermal equator on Mars, the maximum value is 236 K, the minimum is 217 K, and the average is 227 ± 5 K. The northernmost points are at ∼20°N and the southernmost points are at ∼30°S. The average latitude is 7°S ± 12° (averaged over all 7200 longitudes and averaged over both branches in the bifurcated region).

The cause of the bifurcation of the thermal equator appears to be a combination of elevation increasing from north to south while the surface albedo is decreasing from north to south. The northern branch centers at 10°N at an elevation of ∼−2250 m, while the southern branch centers at 20°S at an elevation of ∼+1750 m—both elevations are the Mars Orbiter Laser Altimeter (MOLA) elevations referenced to the mean surface elevation (Smith et al. 2001). The albedo of the northern branch is 0.4, while the albedo of the southern branch is 0.3 (Christensen et al. 2001).

It is interesting to note that the thermal equator runs through Valles Marineris, which is a topographic low, but it also has high thermal inertia—almost twice that of the surrounding terrain (Mellon et al. 2000, Fig. 2). As the examples discussed above show, the thermal equator on Mars depends on elevation, thermal inertia of the surface, and albedo, and it is not simply correlated with any one of these. It may also depend on the orbital eccentricity of Mars which creates significant asymmetries in the seasons (e.g., Richardson and Wilson 2002).

4. Conclusions

We have analyzed recent high-resolution datasets for the mean annual temperature on Earth and Mars to construct the thermal equator on both worlds. From this analysis, we draw the following conclusions.

On Earth, the thermal equator is not well represented by a constant latitude nor by a great circle. Our analysis indicates that while the thermal equator is well represented by a thin line over about half of Earth’s surface, it is a zone of varying thickness in other locations. While the thermal equator is generally within ±20° of the equator, it varies greatly as influenced by the location of the continents. Thus, on a world with no oceans or continents, the thermal equator may be simpler. The range of mean annual temperatures along the thermal equator averages 27.75° ± 1.3°C.

A precise knowledge of the location of the thermal equator may be of use in tropical ecology. The clearest example may be the elevation of treeline in the tropical mountains of the Americas (Körner 2021a,b). For example, studies of the high treeline, 4000 m, in Pico de Orizaba in Mexico (Cruz-Kuri et al. 2001) must consider that despite being at 19°N, it is only 4°N of the thermal equator. Similarly, the treeline, 4000 m, in the Sierra Nevada de Mérida in the Venezuelan Andes 8°32.5′N (Marcano et al. 2024) is north of the equator but is 3°S of the thermal equator at that longitude. In the ocean, the precise location of the thermal equator may be of interest in tropical marine ecology, such as coral reef survival. For these reasons, we include a digital listing of the thermal equator on Earth and Mars available online.

For Mars, lacking oceans, the thermal equator is simpler than the thermal equator on Earth. The Martian thermal equator is roughly parallel to the equator and displaced 5°–10°S. However, the Martian thermal equator bifurcates into two distinct branches, resulting from a combination of topography and albedo.

Acknowledgments.

This research is part of an ongoing effort in studies of life in extreme environments supported by the NASA Astrobiology Program. MNC acknowledges a NASA summer internship. The authors declare no competing interests. We thank the reviewers for the extensive comments that greatly contributed to the clarity and presentation of this work.

Data availability statement.

Data analyzed in this study for Earth and Mars were openly available in digital form through the references cited. A digital listing of the thermal equator on Earth and Mars is available online at https://zenodo.org/records/10460684. Maps used in Figs. 24 are available from NASA without copyright restriction.

References

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Save
  • Adam, O., T. Bischoff, and T. Schneider, 2016: Seasonal and interannual variations of the energy flux equator and ITCZ. Part I: Zonally averaged ITCZ position. J. Climate, 29, 32193230, https://doi.org/10.1175/JCLI-D-15-0512.1.

    • Search Google Scholar
    • Export Citation
  • Boos, W. R., and R. L. Korty, 2016: Regional energy budget control of the intertropical convergence zone and application to mid-Holocene rainfall. Nat. Geosci., 9, 892897, https://doi.org/10.1038/ngeo2833.

    • Search Google Scholar
    • Export Citation
  • Broecker, W. S., and A. E. Putnam, 2013: Hydrologic impacts of past shifts of Earth’s thermal equator offer insight into those to be produced by fossil fuel CO2. Proc. Natl. Acad. Sci. USA, 110, 16 71016 715, https://doi.org/10.1073/pnas.1301855110.

    • Search Google Scholar
    • Export Citation
  • Chavez, F. P., A. Bertrand, R. Guevara-Carrasco, P. Soler, and J. Csirke, 2008: The northern Humboldt Current System: Brief history, present status and a view towards the future. Prog. Oceanogr., 79, 95105, https://doi.org/10.1016/j.pocean.2008.10.012.

    • Search Google Scholar
    • Export Citation
  • Christensen, P. R., and Coauthors, 2001: Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results. J. Geophys. Res., 106, 23 82323 872, https://doi.org/10.1029/2000JE001370.

    • Search Google Scholar
    • Export Citation
  • Cruz-Kuri, L., C. P. McKay, and R. Navarro-Gonzalez, 2001: Spatial and temporary patterns of some climate parameters around the timberline of Pico de Orizaba. First Steps in the Origin of Life in the Universe, Springer Netherlands, 293301.

    • Search Google Scholar
    • Export Citation
  • Hatchett, B. J., 2018: Fingerprints of the thermal equator. Nat. Geosci., 11, 387, https://doi.org/10.1038/s41561-018-0129-1.

  • Kang, S. M., I. M. Held, and D. M. W. Frierson, and M. Zhao, 2008: The response of the ITCZ to extratropical thermal forcing: Idealized slab-ocean experiments with a GCM. J. Climate, 21, 35213532, https://doi.org/10.1175/2007JCLI2146.1.

    • Search Google Scholar
    • Export Citation
  • Kang, S. M., D. M. W. Frierson, and I. M. Held, 2009: The tropical response to extratropical thermal forcing in an idealized GCM: The importance of radiative feedbacks and convective parameterization. J. Atmos. Sci., 66, 28122827, https://doi.org/10.1175/2009JAS2924.1.

    • Search Google Scholar
    • Export Citation
  • Karger, D. N., and Coauthors, 2017: Climatologies at high resolution for the Earth’s land surface areas. Sci. Data, 4, 170122, https://doi.org/10.1038/sdata.2017.122.

    • Search Google Scholar
    • Export Citation
  • Kieffer, H. H., 2013: Thermal model for analysis of Mars infrared mapping. J. Geophys. Res. Planets, 118, 451470, https://doi.org/10.1029/2012JE004164.

    • Search Google Scholar
    • Export Citation
  • Körner, C., 2021a: The cold range limit of trees. Trends Ecol. Evol., 36, 979989, https://doi.org/10.1016/j.tree.2021.06.011.

  • Körner, C., 2021b: Alpine Plant Life: Functional Plant Ecology of High Mountain Ecosystems. 3rd ed. Springer-Verlag, 344 pp.

  • Körner, C., and J. Paulsen, 2004: A world‐wide study of high altitude treeline temperatures. J. Biogeogr., 31, 713732, https://doi.org/10.1111/j.1365-2699.2003.01043.x.

    • Search Google Scholar
    • Export Citation
  • Marcano, V., R. Navarro-Gonzalez, and C. P. McKay, 2024: Temperatures and treeline elevation in the Sierra Nevada de Mérida of the Venezuelan Andes. Arct. Antarct. Alp. Res., in press.

    • Search Google Scholar
    • Export Citation
  • Mellon, M. T., B. M. Jakosky, H. H. Kieffer, and P. R. Christensen, 2000: High-resolution thermal inertia mapping from the Mars global surveyor thermal emission spectrometer. Icarus, 148, 437455, https://doi.org/10.1006/icar.2000.6503.

    • Search Google Scholar
    • Export Citation
  • Putnam, A. E., and W. S. Broecker, 2017: Human-induced changes in the distribution of rainfall. Sci. Adv., 3, e1600871, https://doi.org/10.1126/sciadv.1600871.

    • Search Google Scholar
    • Export Citation
  • Putnam, A. E., and Coauthors, 2012: Regional climate control of glaciers in New Zealand and Europe during the pre-industrial Holocene. Nat. Geosci., 5, 627630, https://doi.org/10.1038/ngeo1548.

    • Search Google Scholar
    • Export Citation
  • Ramage, C. S., and A. M. Hori, 1981: Meteorological aspects of El Niño. Mon. Wea. Rev., 109, 18271835, https://doi.org/10.1175/1520-0493(1981)109<1827:MAOEN>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Richardson, M. I., and R. J. Wilson, 2002: A topographically forced asymmetry in the Martian circulation and climate. Nature, 416, 298301, https://doi.org/10.1038/416298a.

    • Search Google Scholar
    • Export Citation
  • Smith, D. E., and Coauthors, 2001: Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars. J. Geophys. Res., 106, 23 68923 722, https://doi.org/10.1029/2000JE001364.

    • Search Google Scholar
    • Export Citation
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  • Wright, S. J., 2016: Tropical flowering phenologies. 2016 Fall Meeting, San Francisco, CA, Amer. Geophys. Union, Abstract B41I-07.

  • Fig. 1.

    The mean annual temperature on Earth from 1981 to 2010. Data are from Karger et al. (2017) GeoTIFF file CHELSA_bio1_1981–2010_V.2.1. The temperature resolution of the data is 0.1°C and the spatial resolution is 0.0083°, extracted from the GeoTIFF format using the RasterIO programming package in Python.

  • Fig. 2.

    The thermal equator. As a function of longitude: (a) the maximum annual average temperature, (b) the extent in latitude, and (c) average location. The base topographic map is from the NASA Shuttle Radar Topography Mission.

  • Fig. 3.

    Enlargement of the region near Australia but now with all pixels with mean annual surface temperatures equal to the maximum for that longitude marked red. In this area, the thermal equator is well represented as a band of latitude, ∼30° wide.

  • Fig. 4.

    The thermal equator on Mars showing the average position at each longitude of the locations with the maximum mean annual temperature at that longitude. Over the longitude range 135°E eastward to 90°W, the thermal equator has two separate branches.

  • Fig. 5.

    The bifurcation of the Mars thermal equator from Fig. 4 shown with the temperature data of Kieffer (2013, Fig. 10) in the background. Temperature scale: pink: 200 K, blue: 208 K, green: 216 K, and yellow: 222 K.

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