Today is the winter solstice - "the shortest day of the year". This weekend marks the time of year when hours of daylight are shortest and hours of darkness are longest.

For a home like ours that is powered mostly by sunlight energy, this is not good news.

But every cloud has a silver lining. Here's what I mean.

Although the end of June marks the time when hours of daylight are shortest, it is not necessarily the coldest time of year - that comes later.

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In other words, as June turns to July and temperatures drop on average, the days actually get longer.

This may sound counterintuitive: more sun but colder. What's up with that?

It all has to do with lag time, or what may also be called thermal momentum or seasonal inertia. Put simply, there is a delay in the system between energy input (amount of sunlight) and how we experience that energy (air temperature).

Most of the seasonal delay is influenced by large bodies of water: oceans, seas, very big lakes. These large bodies of water are the thermal mass of the planet - they absorb heat slowly and release it slowly. Sunlight energy is loaded into the world's waters only for it to be released at a later date.

On a very large scale, most climate scientists say that much of the excess heat energy that the Earth is currently absorbing is going into the world's oceans. They refer to oceans as "heat sinks". The major concern with this situation is that the "sinks" will become "sources" in the future. In other words, the chickens (massive amounts of heat energy) will come home to roost (wreak havoc on us with extreme weather events).

While this energy is being stored in the oceans, everything appears to us to be okay. It is a lot like running up a large debt. I suspect there were few complaints in Wanganui while the council was running up our current debt while holding rates artificially low. Only now do we hear complaints.

This is the same strategy that US President George Bush (the second) used with the Iraq War. He did not tax Americans to pay for the war but put it on the national credit card. There were few complaints at the time but now, after a trillion dollars, we hear complaints about the "unsustainable levels of federal debt" in the United States.

Similarly, climate scientists continue to warn of "unsustainable levels of carbon debt," but I suspect more and more people will echo them in the future, especially because another and perhaps more ominous delay is also built into the climate system.

Once fossil fuels are burned, the carbon dioxide remains in the atmosphere for decades, causing more and more warming. Many scientists say that, even if we stopped burning all coal, oil and gas today, we would continue to experience the effects for the better part of most Chronicle readers' lifetimes.

In the same way, even if WDC balanced the city's budget next year, we will all still be paying for debts racked up in the past and the accrued interest for years to come.

Okay, now for the silver lining ... for our house anyway. Heading into July and through August, as temperatures remain low, the increasing minutes of sunlight every day make our solar home that much warmer. Additionally, we use a "delay system" inside our home to capture the daytime warmth and release it at night.

This delay is, of course, thermal mass and it acts just like the Tasman Sea outside our front door: absorbing heat slowly when it is in abundance and releasing it slowly when it is in deficit.

Understanding complex systems and their associated delays, oscillations, changes and feedback loops helps us to "see" into the future and plan accordingly. This way of seeing the world is called "systems thinking" and is at the heart of eco-design. It has helped us design and renovate an inefficient old villa into a low-energy eco-home, and it has the potential for humanity to come to grips with global climate change and unsustainable debt.

Human beings are notoriously bad at looking towards the future and planning ahead. Systems thinking is a tool to help us all look towards an increasingly volatile and indebted future, ask if it is the future we want for our children, and then decide whether we have the courage to do anything about it today.