Closer to the Fire – Increasing Tropical Sunlight

Richard Willoughby

Summary

This article analyses long-term changes in tropical solar energy arising from orbital precession and their role in driving atmospheric moisture, poleward heat transport, and global temperature evolution. It indicates that increasing tropical solar exposure enhances convective energy throughput, amplifying total precipitable water (TPW), poleward heat export, and ocean heat retention outside the tropics.

The purpose of this analysis is to examine whether changes in tropical solar exposure are consistent with observed changes in atmospheric moisture and heat redistribution.

This analysis does not indicate any trend in global solar forcing, but rather, examines how changes in the timing and geographic distribution of tropical solar energy affect the operation of the convective climate system.

Precession of Perihelion

The current precession cycle that defines the Holocene epoch resulted in perihelion first re-aligning with the December solstice in 1143AD with zenith solar declination at 23.55S.  Perihelion regularly aligned with the December solstice till 1345 when zenith declination was 23.52S.  Accordingly, 1255 was selected as the base year for this analysis because it was close to the middle of the period as well the December solstice and perihelion being concurrent.  Zenith declination was 23.53S in that year with intensity of 1409.2W/m².   

The daily average solar intensity was calculated over the period from 1200 to 1500 to produce a cumulative anomaly relative to 1255.  Chart 1 shows the result.

The 2^nd order trend line indicates that 1255 is indeed very close to the bottom of the solar energy available over the tropics in the present precession cycle.  It is important to observe from the chart that 1255 is closely aligned with reversal of the available solar energy in the tropics.  However, the varying thermal response of elements in the climate system and changing solar intensity preclude any state of thermal equilibrium at any point in time.   So 1255 simply represents the most likely year that the trend in tropical solar energy reached an inflection.   In 1255 the Equator was the furthest it will be from the Sun in this precession cycle when the Sun is directly over the Equator.  The Sun-Earth distance at both March and September equinoxes were almost the same.  For the next 5,000 years, the Sun-Earth distance at the March equinox will reduce till perihelion is concurrent with the March equinox during the 7^th millennium. 

Base Case Top of Atmosphere Solar Intensity and Solar Energy

The calculation of average daily solar intensity is based on orbital data from NASA/JPL.  The calculation provides the daily average top of the atmosphere (ToA) solar electro-magnetic radiation (EMR) available to heat Earth’s atmosphere and surface.  Not all EMR is thermalised.  Some is reflected but this analysis is based entirely on what solar EMR is available at the top of the atmosphere.  Image 1 provides a pictorial display of the available average daily solar EMR across the tropics from 30S to 30N.  The image is a 60X365 color coded matrix.

The solar intensity ranges from 207W/m² up to 497W/m².  The boundaries between the warmer and cooler zones are shown at 425W/m² as this has been observed as the ToA intensity required to produce convective overshoot that marks the transition to cyclic convective instability that gives rise to deep convection and resulting monsoon.

The Present Era

By 2024, perihelion was some 12 days later than the December solstice when zenith declination had reduced to 22.89S.  This small change in timing of perihelion and declination at perihelion has a noticeable change on the distribution of solar intensity and the overall available energy as shown in Image 2.

The daily average solar intensity in 2024 ranged from 208W/m² to 497W/m².  The changes compared with 1255 are easier to observe by looking at the differences per Image 3.

The top panel color codes the daily difference in solar intensity across the tropics.  The greatest increase is at 30N in April and the greatest decrease is also at 30N but in October.  The annual energy available has increased by almost 4 ZJ across the tropics based on these calculations, with approximately half of this change attributable to orbital precession, reducing obliquity and reducing eccentricity.

The lower panel of Image 3 displays the zones that are now above 425W/m² but were not in 1255 and those now below 425W/m² but were in 1255.  The total annual area above 425W/m² has increased by 0.3%. The timing has changed noticeably as shown in the image.  In 1255, there were two distinct period of monsoon with delays between hemispheric transition.   In 2024, the Southern Hemisphere (SH) monsoon is still active when the Northern Hemisphere (NH) monsoon is commencing.  This is most noticeable in the Indian Ocean and western Pacific by increasing ocean surface area reaching 30C in April with comparatively much lower area reaching 30C in October.

Note: The 425 W/m² threshold was derived empirically from ocean moored buoy observations by comparing sustained daily-average ToA solar input with the onset of persistent deep convection and associated SST behaviour near 30°C.  The moored buoy at 15N 90W in the Bay of Bengal provides the best observing platform for cyclic deep convection due to this region sustaining above 425W/m² for typically 157 days in the present era.  Globally, 425W/m² is observed as an inflection point in ocean surface temperature where the temperature peaks.  ToA solar intensity above that level is associated with declining surface temperature per Chart 1A.

Chart 2 gives insight into why the NH is experiencing similar solar energy to the SH despite higher peak solar intensity and higher maximum daily average in the SH.  In 1255 the Sun zenith transitioned to the NH on day 80 then transitioned to the SH on day 267.  So the Sun zenith was over the NH for 187 days and the SH for only 179 days. 

Chart 3 shows how the solar intensity has changed at 30S and 30N from 1255 to 2024.

The NH is undergoing much larger changes in solar intensity than the SH.  There is a significant increase in March and April solar intensity but a significant reduction in September and October. NH overall has a slight increase in average.  The SH has almost identical increase in average sunlight but it is more evenly spread through the autumn and winter months and a small reduction in October – November sunlight.

Increased Sunlight & Atmospheric Water

There have been global satellite measurements of total water column since the late 1980s.  Chart 4 provides the yearly difference in total precipitable water (TPW) over oceans for 2024 using 1990 as the base.

The increase of 2.3mm over half the area of the globe would require 1.4 ZJ for evaporation.  Chart 5 shows the cumulative difference in solar energy input taking 1990 as the reference year rather than 1255.

As shown, the cumulative tropical solar energy above the 1990 level is 28 ZJ.   That is 20 times more than the latent heat that is required to increase the atmospheric water by 2.3mm.  However, there are many factors that limit the uptake of the available energy and consequent increase in TPW; including:

• Regions that experience monsoon reject available solar EMR by producing reflective cloud.  Little to no solar EMR above 425W/m² even enters the lower troposphere over open oceans due to reflection from high altitude cloud.
• Taking a base year as a reference should not presume that the system is in energy equilibrium in that year.  It is obvious that energy equilibrium can never be the situation on Earth due to the continually varying orbit and changing solar activity in combination with vast array of thermal response and thermal lag of all the elements that form the climate system.
• Getting moisture into the atmosphere in the tropics requires much more energy than the latent heat.  Increasing the TPW requires additional sensible heat for atmospheric expansion beyond the latent heat of evaporation, increasing the total energy requirement well beyond the latent heat for a given TPW change.
• Once the moisture is in the atmosphere it increases absorption of solar EMR and long wave radiation so sensible heat increases to maintain or even reduce relative humidity and the higher energy atmospheric column can then transport more heat poleward.  Moisture increase in the tropics also causes moisture increase at higher latitudes that reduces heat loss from the oceans in the mid latitude due to precipitation supressing evaporation.    

The increasing robustness of transition from SH monsoon to NH monsoon is particularly noticeable in the increasing trend in area of Ocean surface reaching 30C in April and then the increased monsoon activity that follows in July.  July is the month of greatest tropical moisture increase and is most apparent just north of the Equator per Chart 6 where the increase in TPW is 9 mm in July 2024 wrt July 1990.

This highlights that the climate response is controlled by how energy is processed within the convective system, rather than just the absolute magnitude of solar input alone.

Precession in the Future

The two following Images 4 and 5 are previews of the changing solar intensity and annual energy available over the tropics early in the next millennium and the 10^th millennium when zenith at perihelion will be well north of the Equator.

In 3065, perihelion solar intensity will be 1408.2W/m² and occur over 19.88S.  The solar intensity at 30N in March will be 10.4W/m² above 1255 and 5W/m² above current level.  Solar intensity at 30N in September will be 10W/m² below the level in 1255.  The available solar energy over the tropics will be 6.8 ZJ above the 1255 level.  The SH has lower solar intensity in summer and higher in winter, but the atmospheric moisture will still be increasing due to the increase in tropical solar energy.

The zenith at perihelion moves north of the Equator in the 7^th millennium and the zenith will be over 16.12N at perihelion in 9038.  By then there will be noticeable changes in the monsoon with it increasing in both time and extent in the NH.  The largest change in solar intensity will then be at 30S; down by 27.8W/m² relative to 1255.  It is interesting that the available solar energy will continue to increase due to the eccentricity of the orbit reducing as well as obliquity reducing.  The zenith solar intensity at perihelion will be down to 1400.2W/m².

By 9038, the extremes in solar intensity will be greater in the NH than the SH but still well short of what is currently experienced in the SH. 

Overall, orbital evolution continues to cause upward trend in tropical energy availability through increase in total energy and changes in seasonal distribution, rather than large increases in peak intensity or total energy.

Discussion

Tropical solar energy is a key climate variable – The climate system is driven by the tropical energy surplus. Net solar input is greatest over low-latitude oceans, and this excess energy drives atmospheric and oceanic heat transport toward higher latitudes. The relevant climate solar forcing is therefore not global mean radiation, but the area and duration of tropical ocean exposed to solar flux sufficient to trigger persistent cyclic deep convection that powers the circulation system. The analysis shows that this tropical solar exposure has increased since perihelion and the December solstice were aligned in 1255, which is used here as the baseline. This represents a small direct increase in available tropical solar energy and more significant changes in spatial and seasonal exposure while global mean flux only changes in line with solar activity.

Convective threshold and SST regulation – Observations from ocean moored buoys indicate that sustained tropical convection develops when daily-average ToA solar flux exceeds ~425 W/m² for sufficient time for surface temperatures to reach ~30 °C. Once this threshold is maintained, evaporation accelerates, deep convection initiates, and atmospheric moisture increases. At this stage, further energy input does not produce unconstrained SST rise. Instead, convective regulation dominates.

This regulation is associated with convective overshoot once surface temperatures exceed ~30 °C, leading to the formation and persistence of micron-scale ice particles in the upper troposphere, increasing shortwave reflection. Together with enhanced evaporation and latent heat export, this acts as a strong regulatory process limiting further SST increase while maintaining high energy throughput.  Deep convective overshooting injects moist air into the very cold upper troposphere, where rapid freezing produces large numbers of micron sized ice crystals that are detrained into cirrus anvils and above-anvil plumes, forming persistent high-altitude ice clouds.  Cyclic deep convection operates below the onset of convective overshoot, which is often observed when regional deep convection is initiated causing daily average outgoing long wave radiation to drop as low as 130W/m² and daily average reflected short wave to increase by ~200W/m².   The net effect is a reduction in short wave radiation reaching the surface so the surface cools.

Increased TPW as the primary amplification response – The first-order response to increased tropical solar energy is increased evaporation and total precipitable water (TPW), rather than SST.  Rising TPW increases energy throughput in the convective system by enhancing latent heat transport and contributing to cloud-radiative processes. The consequence is most apparent in July, as shown in Chart 6. Increasing TPW is therefore a direct indicator of increasing tropical energy processing, rather than simply a by-product of temperature change.

Heat export and ocean heat retention – The tropics act as a heat source region rather than the primary heat storage region. Once energy enters the convective regime, heat is exported via atmospheric circulation, with latent heat released at higher latitudes. Ocean heat retention consequently increases preferentially outside the tropical source region.

This is consistent with the pattern observed in Chart 7, which shows enhanced ocean heat retention in the latitudes of the Ferrel cells during the ARGO era. These regions represent zones of moisture convergence, condensation, and reduced ocean convective heat rejection, meaning heat can be more effectively retained within the ocean system.

Increased precipitation associated with elevated TPW contributes to this process by enhancing near-surface freshening and stratification. This promotes strengthening of upper-ocean barrier layers, which can suppress vertical mixing and reduce heat exchange between the surface mixed layer and the thermocline.   The result is a tendency toward increased heat retention in mid-latitude regions, even while the tropics remain strongly regulated by atmospheric deep convection.

Further Studies – The developing asymmetry of monsoon transitions based on observation of solar intensity above 425W/m² and increasing area of ocean above 30C in April give rise to questions yet to be answered conclusively that are consistent with the shifting monsoon.

Observations over the Indo‑Pacific warm pool show that the SH→NH transition is generally coherent, with convection expanding northward from the warm pool into the developing Asian monsoon. The reverse NH→SH transition is markedly less stable, with convection shifting between hemispheres, observed double‑ITCZ states, and rapid changes in the dominant convective centre.  This asymmetry reflects a stable buildup phase in the NH versus an unstable decay phase, as competing convection from the Indian Ocean, western Pacific, and Atlantic warm pool reorganises the Walker circulation.  Because this occurs across the core El Nino-Southern Oscillation (ENSO), the unstable NH→SH transition provides a natural mechanism for perturbing trade winds and convection, potentially contributing to Pacific oscillation variability.

Conclusions

Common Wisdom – The prevailing view within the scientific community is that total solar forcing is approximately constant and that tropical insolation changes are primarily redistributive rather than amplifying. However, this framing underestimates the importance of both long-term changes in available solar EMR and non-linear behaviour in tropical convection. Even modest changes in tropical solar exposure can alter evaporation, total precipitable water (TPW), atmospheric absorption of short wave and long wave, cloud-mediated shortwave reflection, and poleward heat transport. The critical variable is therefore not global mean available solar EMR, but the extent and persistence of tropical regions operating within the convective regime.

The Warming Trend – The observed warming is not uniquely a recent phenomenon but is consistent with a longer-term evolution linked to changes in tropical energy availability.  Long climatological records and proxy evidence show multidecadal and centennial trends extending well prior to the modern era, while satellite observations show a sustained upward trend with substantial variability, rather than a response dominated by recent acceleration.

This behaviour is consistent with a system responding to gradual changes in tropical solar exposure and increasing convective energy throughput. As tropical regions more frequently exceed the convective threshold, additional solar input is not expressed as unrestricted surface warming, but is redistributed through enhanced evaporation, increased atmospheric moisture, increased short wave and long wave atmospheric absorption, cloud-mediated shortwave rejection, and poleward heat transport.

The resulting response is structured warming in which energy is redistributed and retained outside the tropical source region. Variability is superimposed through solar variability and internal circulation dynamics, rather than representing a purely recent or step-change forcing. With this insight, the modern temperature record is best interpreted as the current phase of a longer-term response to evolving tropical solar energy and its amplification through moisture transport and convective heat export.

Physically Consistent – These observations provide a physically consistent explanation for the observed separation between tropical energy input and mid‑latitude heat retention. They reinforce the role of the tropics as the primary control region of the climate system and highlight the importance of convective regulation and moisture-driven heat transport in shaping global climate evolution.

Data Sources and Methods

All datasets are publicly available through NASA, NOAA, and international observing system portals, ensuring reproducibility.  The author acknowledges the contribution of the USA Government and the supported scientific community in producing the data and making it publicly available.

Orbital and Solar Geometry – Orbital parameters and Earth–Sun geometry were taken directly from the NASA Jet Propulsion Laboratory (JPL) ephemeris system (NASA JPL, 2025). The JPL Horizons database provides high-precision time-dependent values for heliocentric Sun-Earth distance and solar declination and Sun velocity vectors.  All data used had a time increment of one day.

Solar Irradiance and Solar Activity – Solar irradiance variability was inferred from the square of the Sun orbital velocity variation from its average with a 2.4 year phase shift and validated against SORCE project TSI measurements.   TSI measurements indicate variability range on solar-cycle timescales of approximately 0.2%, which is noticeable over decadal periods but less significant over millennial trends.

Computation of Daily Mean Insolation – Daily-mean solar radiation at the top of the atmosphere (ToA) was calculated using established solar-geometry relations (Berger, 1978; Hartmann, 1994). The instantaneous solar flux is expressed as:

where $S_0$ is the solar constant, $d$ is the instantaneous Earth–Sun distance, and $θ_s$  is the solar zenith angle (Hartmann, 1994).

The zenith angle is given by:

where $ϕ$ is latitude, $δ$ is solar declination, and $h$ is the hour angle (Hartmann, 1994).

Daily-average insolation is obtained by integrating the instantaneous flux over the daylight period defined by the sunrise and sunset hour angles. The formulation follows the analytical solution presented by Berger (1978), which provides a complete framework for calculating daily insolation as a function of orbital parameters and latitude.

Tropical Solar Energy Calculation – Daily-average ToA solar intensity was calculated for each day of the year across the latitude band 30°S–30°N at 1-degree intervals. The resulting dataset is a latitude–time matrix of daily ToA solar radiation. Annual energy was obtained by integrating the daily flux over the area at each latitude for each year with all years taken as 365 days to avoid the step introduced by an extra day every 4 years.  The start day of the year is not always January 1^st. Rather it is the nearest whole day based on a year having 365.2422 days.  This reduces annual variation that results from using 3 X 365 day years and 1 X 366 day year. (A problem observed when integrating the CERES ToA monthly sunlight data over a year)

Validation Against Satellite Observations – The calculated ToA solar radiation fields were validated against satellite observations from the NASA CERES (Clouds and the Earth’s Radiant Energy System) dataset (Loeb et al., 2021). CERES provides globally observed shortwave and longwave radiation fluxes at the top of the atmosphere.

Comparison of calculated and observed fields confirms that the calculation reproduces the spatial distribution and seasonal variability of incoming solar radiation within expected uncertainty bounds, supporting the validity of the applied methodology.

Atmospheric Moisture (Total Precipitable Water) – Total precipitable water (TPW) data were obtained from satellite-based microwave radiometer datasets (Wentz et al., 2007; RSS, 2022). These datasets combine measurements from SSM/I, SSMIS, AMSR‑E, AMSR‑2, and WindSat instruments to produce a consistent global record of oceanic atmospheric moisture from 1988 to present.

TPW represents the vertically integrated atmospheric water column and serves as the primary diagnostic of moisture response to changes in tropical solar energy.

Ocean Heat Content – Ocean heat content (OHC) data were obtained from NOAA Climate Data Records and Argo float observations (Levitus et al., 2012; Johnson et al., 2015).

The Argo program provides a global array of profiling floats measuring temperature and salinity throughout the upper 2000 m of the ocean, enabling high-resolution estimates of ocean heat retention and distribution. These datasets were analysed to identify latitudinal variations in ocean heat retention.

References with Active Links

Orbital and Solar Geometry

• NASA Jet Propulsion Laboratory (JPL). Horizons System Ephemeris Database.
  Access Horizons System

Solar Radiation and Insolation Theory

• Berger, A. L. (1978). Long-term variations of daily insolation and Quaternary climatic changes. Journal of the Atmospheric Sciences, 35, 2362–2367.
  View Berger (1978) paper
• NASA GISS (2020). Solar radiation calculation methods (ModelE).
  View NASA GISS insolation documentation
• Hartmann, D. L. (1994). Global Physical Climatology. Academic Press.
  (Foundational reference; no direct link provided in source results)
• Rose, B. E. J. (University at Albany). Insolation and solar geometry notes.
  View insolation calculation notes

Solar Irradiance

• NASA Earth Science Division. Solar Irradiance Science (TSI datasets).
  View solar irradiance overview

Radiation Budget (Satellite Observations)

• NASA CERES (Clouds and the Earth’s Radiant Energy System).
  Access CERES data and products
• CERES EBAF Dataset (TOA Radiation Climate Data Record).
  View CERES EBAF dataset description

Atmospheric Moisture (Total Precipitable Water)

• Remote Sensing Systems (RSS). Microwave Total Precipitable Water Dataset.
  Access RSS TPW dataset
• NASA Earthdata (GHRC DAAC). Merged Microwave TPW Dataset.
  View NASA TPW dataset description
• NOAA NCEI. SSM/I–SSMIS Hydrological Products.
  View SSMI/SSMIS dataset overview

Ocean Heat Content

• NOAA NCEI. Global Ocean Heat Content Climate Data Record.
  Access NOAA OHC dataset
• Argo Program (Global Ocean Observing System).
  Access Argo data portal

Supporting Climate Data Resources

• NOAA Climate Data Records (CDR).
  Explore NOAA CDR datasets
• UCAR Climate Data Guide. Ocean datasets overview.
  View ocean climate dataset guide

Hemispheric Monsoon Transition (Observations)

• Wang, B. & Ding, Q. (2008). Global Monsoon: Dominant mode of annual variation in the tropics.
  → View Global Monsoon paper
• Gu, G., Adler, R. & Sobel, A. (2005). The Eastern Pacific ITCZ during the Boreal Spring.
  → View Journal of Atmospheric Sciences paper
• Satiadi, D. et al. (2023). Study of ITCZ movement and cross‑equatorial behaviour.
  → View ITCZ movement study
• NOAA. Intertropical Convergence Zone (ITCZ) overview.
  → View NOAA ITCZ description
• NASA GPM. Observed precipitation structure of the ITCZ.
  → View NASA ITCZ observations

Tropical Moored Buoy Observations

• McPhaden, M. J. et al. (2023). Global Tropical Moored Buoy Array overview.
  → View Oceanography Society paper
• NOAA. Global Tropical Moored Buoy Array (TAO/TRITON, PIRATA, RAMA).
  → View NOAA buoy system overview
• NCAR Climate Data Guide. Tropical Moored Buoy System.
  → View buoy array description
• NOAA NDBC. Tropical Atmosphere Ocean (TAO) buoy array.
  → View TAO data portal

Convective Overshoot & Ice Particles

• Lee, K.-O. et al. (2019). Convective hydration in the tropical tropopause layer.
  → View ACP TTL observations paper
• Heymsfield, A. et al. (2005). Homogeneous ice nucleation in deep convection.
  → View JAS microphysics paper

The Author

Richard Willoughby is a retired electrical engineer having worked in the Australian mining and mineral processing industry for 30 years with roles in large scale operations, corporate R&D and mine development.  A further ten years was spent in the global insurance industry as an engineering risk consultant where he developed an enduring interest in natural catastrophes and changing climate.