Variability in insolation over the year

As depicted by the Wikipedia image below, the amount of insolation (or solar irradiance) varies over the year, depending on latitude and on the time of year.

Insolation changes with the seasons and also changes between day and night. In the image below, adapted from Wikipedia, latitude is on vertical axis and time of year is on horizontal axis.

                     The June Solstice in 2021 occurred on June 21, 2021.

Why does the Arctic receive so much sunlight at the June Solstice?

In the Arctic, annual insolation is at its highest at the June Solstice, i.e. June 21 for the year 2021, and June 22 for the year 2022. Around this time of year, the sunlight has less distance to travel through the thinner atmosphere over the Arctic, so less sunlight gets absorbed or scattered before reaching the surface.

In addition, the high angle of the Sun produces long days and sunlight is concentrated over a smaller area. Above the Arctic Circle, the Sun does not set at this time of year, so solar radiation continues all day and night. During the months June and July, insolation in the Arctic is higher than anywhere else on Earth. 

How much sunlight does reach the surface further depends on weather conditions such as clouds and how much heat gets pushed by the wind toward the North Pole. As temperatures have risen over the years, the Jet Stream has become more deformed, increasing the chance that heatwaves over land extend over the Arctic Ocean. Deformation of the Jet Stream can also lead to increasingly strong winds speeding up ocean currents that can abruptly push huge amounts of ocean heat into the Arctic ocean, as further discussed at the post Arctic sea ice July 2022.

This partly explains why the temperature rise is strongest in the Arctic, as illustrated by the image on the right, from the FAQ page, which shows anomalies versus 1951-1980 of up to 3.49°C.

Further contributing to the strong rise in the Arctic are albedo changes, loss of the latent heat buffer, methane over the Arctic and jet stream changes that can cause more heatwaves and more ocean heat moving into the Arctic ocean, as also discussed at this page.


By the year 2025, sunspots are expected to reach the peak in their current cycle (#25). Observed monthly values (black diamonds) are looking stronger than predicted (red line), as illustrated by the image below, adapted from NOAA and showing sunspots through to December 2021.

According to James Hansen et al., the variation in forcing is as much as 0.25 W/m⁻² (from solar minimum to solar maximum).

Insolation changes over thousands of years

Milankovitch Cycles are named after Milutin Milankovitch, who in the 1920s hypothesized that long-term, collective effects of changes in Earth’s position relative to the Sun are acting as drivers of recurring periods in Earth’s long-term climate, also known as glacial–interglacial cycles lasting ~100,000 years.

The current interglacial period is known as the Holocene and started 11,700 years ago.

The prior interglacial period or the penultimate interglacial is known as the Eemian, which lasted from about 130,000 to 115,000 years ago. 

Interglacial periods tend to happen during times of more intense summer solar radiation in the Northern Hemisphere. In the image below, from NOAA, interglacial periods are marked by the vertical orange bands. 

The temperature rise at the end of glacial periods tends to happen more abruptly, as a result of albedo feedbacks and because a colder oceans can hold more CO₂.
During the Eemian, oceans reached levels 6–9 m (20–30 ft) higher than today, according to Hansen et al. who further conclude that maximum Eemian temperature was 1°C above pre-industrial, as illustrated by the image on the right, which uses 1880-1920 as base.

This raises the question, what is pre-industrial? Some studies point at evidence of early people's deforestation contributing to CO₂ in the atmosphere during the Eemian.

Nonetheless, the most significant marker for pre-industrial is the year 3480 BC, since emissions by people from that time have been higher than the amount to negate the natural trend for the temperature to fall, so it makes sense to regard 3480 BC as the base year for pre-industrial. 
Summer insolation on the Northern Hemisphere in red and in langleys
per day (left axis, adapted from Walker, 2008). One langley is 1 cal/cm²
(thermochemical calorie per square centimeter), or 41840 J/m² (joules
per square meter), or about 11.622 Wh/m² (watt-hours per square meter).
In blue is the mean annual sea surface temperature, given as the difference
from the temperature over the last 1000 years (right axis, from Bova, 2021).
The image below uses the year 3480 BC as pre-industrial base year and shows that a 3°C rise is possible by 2026.

[ from earlier post ]


• Solstice - Wikipedia

• June Solstice - Wikipedia

• Insolation, solar irradiance - Wikipedia

• Frequently Asked Questions

• Sunspots - NOAA

• Sunspots 

• Young people’s burden: requirement of negative CO2 emissions - by James Hansen et al.

• Glacial-Interglacial Cycles - NOAA

• Milankovitch Cycles - NASA

• Milankovitch Cycles - Wikipedia

• What is Pre-industrial?

• Is the IPCC creating false perceptions, again?