How Earth's Orbit Shapes the Climate on Long Timescales
Earth's path around the Sun is not a circle. It is an ellipse, and the shape of that ellipse is not fixed. The tilt of Earth's rotation axis, the degree to which the orbit deviates from a perfect circle, and the wobble in Earth's rotational axis all change on timescales of between 20,000 and 200,000 years. Together, these shifts are believed to be the primary driver of the Ice Ages that Earth has experienced repeatedly across its geological history.
Karoff explains the wobble using a childhood toy: "So, these things that you used as a kid that you could spin around. When you start the thing, it will spin around itself. But it will also make this wobbling, slowly moving thing. And that's actually what we see with the Earth rotation." Earth spins once a day, but the axis itself traces a slow wobble over roughly 20,000 years.
The 22-degree tilt of Earth's rotation axis creates the seasons we experience year to year. But as that tilt changes over millennia, the length and intensity of the seasons shift. "The change in the Earth's rotation axis is very, very periodic. So that's 20 something thousand years. Then you have the change in sort of how much an ellipse there was, Earth's orbit around the Sun is and how much is a perfect cycle? And that has a longer time, that's the 200,000 years timescale."
What makes this complex is that these multiple cycles interact with each other. When you combine oscillations with different periods, the resulting pattern loses the clean periodicity of any individual cycle. The tidy 20,000-year ice age rhythm that textbooks sometimes describe is a simplification of a far messier reality.
Why We Were Heading for an Ice Age
By the geological clock, we are due for another Ice Age. The last one ended around 20,000 years ago. Based on orbital mechanics, a cooling trend should, over the next few thousand years, be pushing Earth's climate toward lower temperatures.
It is not. And the reason matters.
"If we left things as they were before we start using fossil fuels, then we would, in a few thousand years into the future, move into a new Ice Age," Karoff explains. Human activity has altered the trajectory of a planetary cycle that has been running on schedule for hundreds of millions of years. The CO2 we have added to the atmosphere has swamped the orbital signal. The wobble is still there, still operating on its ancient schedule, but its effect on temperature is now negligible compared to the warming we have introduced.
What this also means is that the slow orbital cooling we might have expected offers no comfort. Even in a scenario where Ice Age cooling were proceeding as predicted, Karoff is clear that "the amplitude of this will be insignificant to the temperature decrease that we are about to witness" without action on emissions. Natural cycles are real. They are just not running in the direction that would help us now.
Sunspots and the Climate Puzzle
In 1758, a Danish astronomer named Christian Horrebow, director of Rundetårn in Copenhagen, first noted a possible connection between the number of sunspots visible on the Sun's surface and variations in Earth's climate. More than 250 years of investigation have followed, and the connection remains both compelling and poorly understood.
A sunspot is a region on the Sun's surface where a strong magnetic field penetrates outward, reducing the efficiency of energy transport in that area. The result is a darker, cooler patch visible against the surrounding solar disc. Given that, the intuitive prediction would be that more sunspots mean a cooler, dimmer Sun, and therefore a cooler Earth.
The data disagrees. "When there are many spots on the surface of the Sun, the Sun actually becomes a little bit brighter." The reason involves the same magnetic activity that produces sunspots. In areas where the magnetic field is weaker, it creates regions called faculae that allow observers to look slightly deeper into the hotter interior of the Sun. These brighter patches more than compensate for the darker sunspots: "Over a cycle, the Sun is a little bit more bright when there are many strong magnetic fields. So when you have many sunspots."
The correlation between sunspot numbers and Earth's climate is real and observable. The mechanism is not fully resolved. And the magnitude of the effect is small: 0.1% variation in the Sun's brightness over the 11-year sunspot cycle, which Karoff describes as genuinely insufficient to explain significant global temperature changes.
The Problem with Measuring the Sun
To determine whether changes in solar brightness have contributed to the warming observed over the last 200 years, you need reliable measurements. Those measurements are remarkably difficult to obtain.
Ground-based observations are compromised by Earth's atmosphere, where water droplets act as small lenses and distort the signal. The solution was to measure solar brightness from satellites. The problem is that satellite instruments degrade over time, becoming progressively less efficient. And unlike most calibration problems in science, you cannot check your solar measurement instrument against any known reference. "The problem is to measure the brightness of the Sun, you cannot compare to anything. Because when you look up, there's nothing in the sky as bright as the Sun."
The result is that even after decades of satellite observations, the absolute level of energy received from the Sun carries an uncertainty of 10 to 20%. The variation in solar brightness over the sunspot cycle that scientists are trying to detect is 0.1%. The signal is roughly 100 times smaller than the measurement uncertainty.
Cross-referencing multiple overlapping satellites has helped, but has not solved the problem. For periods before the satellite era, which began seriously in the late 1970s, researchers use proxy measures: sunspot records going back more than 400 years, and cosmogenic isotopes extracted from tree rings. These suggest the Sun may have been around 0.5% fainter in the 17th century, which would have had a measurable climate effect. But Karoff is careful, until the evidence is stronger, the safest assumption is that recent solar behaviour resembles what has been observed over the past 50 years.
Treating the Symptoms
If solar variability cannot explain the warming, and orbital cooling is too slow and too weak to counteract it, the question becomes: what can be done? Karoff's answer moves into territory that is gaining serious attention in climate science and policy.
The arithmetic is striking. Blocking half a percent of the sunlight reaching Earth would, in theory, be enough to offset the warming that business-as-usual emissions trajectories would produce by the end of the century. Half a percent is a small number. The means of achieving it are not small.
The options under active investigation include fleets of satellites carrying inflatable sun shields positioned to reflect or absorb a fraction of incoming solar radiation, and ships that spray ocean water into the lower atmosphere to encourage cloud formation, reducing how much sunlight reaches the surface. The EU is now funding serious research into both categories. Karoff describes this as a genuine shift: "I mean, we have talked about this for, I mean, for a long time, but sort of started coming up 15 years ago. And now we actually see the governments of the EU going in and putting money into initiatives to actually look at this more seriously."
The objection is obvious, and Karoff states it plainly: "It's not dealing with the roots, it's dealing with the symptoms, and that's probably why people are reluctant to do it." CO2 remains in the atmosphere. Ocean acidification continues. The intervention would need to be maintained indefinitely, growing in scale if emissions continue to rise. Stop the intervention without having addressed emissions, and the suppressed warming would return rapidly.
What is unknown, and what the current research funding is aimed at understanding, is how these interventions would affect ecosystems, weather patterns, and the chemistry of the atmosphere over the long term. These are not small questions, and they do not have answers yet.
What the Sun Can Tell Us About This Winter
Amid the long-term uncertainty, Karoff's research has produced one finding precise enough to be practically useful on a much shorter timescale. Low solar activity is the best single predictor of cold winters in Northern Europe.
The mechanism runs through atmospheric dynamics rather than direct heating. In years of low sunspot activity, the weather patterns that normally bring mild, wet Atlantic air to Northern Europe become less stable. The climate system can tip into an alternative state where cold air from Siberia flows in from the northeast and locks in place. "We can get into another state where we have cold wind coming down from northeast. That's when we talk about that, we get the Siberian cooling down to us. And then it can get locked there."
The practical implication is that if you want to predict whether Denmark will have a white Christmas, tracking the sunspot count is more informative than looking at most other single variables. The Sun, which cannot explain a 200-year trend in global average temperatures, can tell you a great deal about what kind of winter to expect in Copenhagen. It is a reminder that the same physical system can operate very differently at different scales, and that understanding climate means knowing which timescale you are talking about.
The Sun Is Not an Alibi
The Sun's relationship with Earth's climate is not simple, and the simplifications that circulate in popular debate serve no one. The orbital mechanics driving Ice Ages are real, measurable, and predictable, but they operate on timescales that dwarf anything relevant to the decisions being made today. The sunspot cycle has a genuine and poorly understood connection to climate, but the signal is too small and too uncertain to account for observed warming. Geoengineering options exist and are increasingly being taken seriously, but they treat a symptom while the underlying cause accumulates.
What Christoffer Karoff's work suggests, taken as a whole, is that the Sun matters to Earth's climate in ways that are both more specific and more limited than commonly assumed. More specific, because low solar activity genuinely does predict cold Northern European winters. More limited, because no credible reading of the solar data explains the warming of the last hundred years.
The science of the Sun is not an alibi. It is a discipline that rewards careful thinking about timescales, and that is precisely what the current climate debate needs.