Explain how Milankovitch cycles alter insolation and drive long-term climate change

What this dot point is asking

SCSA wants you to explain the three orbital cycles and how they drive long-term natural climate change. The crucial idea is that these cycles change the timing and distribution of sunlight rather than the Sun's output, and that feedbacks turn small changes into large glacial cycles.

The three cycles

• Eccentricity is the shape of Earth's orbit, which slowly shifts between more circular and more elliptical over roughly 100,000 years. A more elliptical orbit changes how much the Earth-Sun distance, and therefore sunlight, varies through the year.
• Axial tilt (obliquity) is the angle of Earth's axis, varying between about 22 and 24.5 degrees over roughly 41,000 years. Greater tilt produces stronger seasonal contrasts.
• Precession is the slow wobble of the spinning axis, on a roughly 23,000-year cycle, which changes the time of year at which Earth is closest to the Sun.

How orbital change drives ice ages

The cycles control glaciation mainly through high-latitude summer sunlight in the Northern Hemisphere, where most landmass capable of holding ice sits.

• When summers are cool because of the orbital configuration, winter snow does not fully melt and ice sheets grow.
• When summers are warm, ice sheets shrink and an interglacial begins.

Acting together and overlapping, the three cycles create the complex but rhythmic pattern of glacial and interglacial periods recorded in ice cores and ocean sediments.

Feedbacks amplify the small push

The change in sunlight from orbital cycles is too small by itself to cause full ice ages. Feedbacks amplify it.

• Ice-albedo feedback: growing ice reflects more sunlight, cooling further and growing more ice.
• Carbon dioxide feedback: colder oceans absorb more carbon dioxide, lowering the greenhouse effect and amplifying cooling, and the reverse on warming.

These feedbacks turn a modest orbital nudge into the large temperature swings between glacials and interglacials.

Why today is different

Orbital cycles operate over tens of thousands of years and cannot explain the rapid warming of recent decades. Milankovitch forcing currently points toward very gradual cooling, so the present rapid rise must come from another cause, the human-driven increase in greenhouse gases, which sets up the anthropogenic climate change content.

The lag of carbon dioxide and a common misconception

Ice-core records show that during past glacial cycles, temperature and carbon dioxide rise and fall almost together, but careful analysis reveals carbon dioxide sometimes lags slightly behind the initial temperature change. This is sometimes wrongly used to argue that carbon dioxide cannot drive warming. The correct interpretation, which SCSA rewards, is that orbital forcing provides the initial trigger (a small change in high-latitude summer sunlight), and as the oceans warm they release dissolved carbon dioxide, which then acts as a powerful feedback that amplifies and globalises the warming. So carbon dioxide is both an effect (released by warming oceans) and a cause (its greenhouse warming drives further temperature rise) in the past cycles. Crucially, in the present day the order is reversed: humans are adding carbon dioxide first, so it is the initial driver, not a lagging feedback. Understanding carbon dioxide as an amplifying feedback in the orbital cycles, distinct from its role as the leading cause today, is a hallmark of a strong response.