Not only does climate change affect the production and distribution of food, it undermines people’s livelihoods and destabilizes food prices.

This week marks a critical point in measuring the severity of global climate change and associated impacts. The Intergovenmental Panel on Climate Change (IPCC) are compleing their updated report on the physical science behind climate change. This is the first comprehensive scientific update by the IPCC since 2007 and it will lay out a series of scenarios against which to measure ourselves as we work towards implementing the President’s Climate Action Plan.

The growing global climate change crisis can only be addressed in the context of persistent poverty and food insecurity in much of the world. In a post today entitled, “A hotter world is a hungrier world”, my colleague John Magrath links recent climate-related events to income and food insecurity globally. Check it out below, as well as Oxfam’s new policy brief, Growing Disruption: climate change, food and the fight against hunger.

The main impact of climate change on food supplies is often considered to be how rising temperatures, erratic rainfall, and climate “shocks” will directly damage crops, wiping out harvests and directly depriving poor farmers of the food that they grow to feed themselves and their families.

And indeed, we should be very worried about those impacts.

But thinking only about how climate shocks ruin crops can make us forget how most people actually obtain most of the food they eat; they don’t grow it for themselves, they buy it. Even smallholder farmers rarely obtain all of the food they need from their own production. They too buy food on the market, trading their earnings or what they grow for types of food that they can’t grow themselves.

Everyone, therefore, needs some income to buy some food. (And not only food of course, but they need money to pay for kids to go to school, to pay medical expenses and inevitably to pay taxes.)

So what happens if, as a result of climate change, suddenly your income drops by a quarter, or even a half? Even if food is still as available as ever in the market, your capacity to buy it has shrunk massively.

In Pakistan the devastating 2010 mega-flood destroyed over 570,000 hectares of crop land in Punjab and affected more than 20 million people. Eighty per cent of food reserves were lost. The destruction of crops and drowning of animals meant not only that people had nothing to eat, but that they had nothing to trade to be able to buy food as it became available.

The flood caused a massive 75% reduction in income across all households affected. And more floods have followed every year since. Recently the World Food Programme warned that three consecutive years of floods have left about half of Pakistan’s population without “secure access” to enough food, which is up from a little over one-third a decade ago.

Or take the current situation in Central America. There a fungal disease of coffee called La Roya has suddenly gone berserk, infecting coffee plants across Guatemala, Honduras, El Salvador and Nicaragua. Coffee rust has been in the region for years, but its exceptional spread this year – including to higher-altitude coffee farms and plantations hitherto thought immune to the fungus – has been blamed in part on several years of above average temperatures and rainfall, creating the ideal humid conditions for the spores to spread.

Big farms can invest in prevention and control, including using more fungicides. But many small farmers face ruin. Their best tactic is to uproot sick bushes and plant new ones; but the new ones will not yield coffee beans for at least three years.

However, the most devastating impacts most likely will not be on coffee farmers themselves but on the 1.4 million unskilled laborers who depend on wages from coffee harvesting for much of their annual income. The statistics are truly frightening; daily income from the coffee harvest labor is likely to be up to 50% below average this year (2013/14) and next. Oxfam programs are gearing up to help.

Income insecurity has all sorts of knock-on effects. People may reduce their expenditure on not just food, but other important aspects of their lives, such as their children’s education, medical expenses, heating bills. In Russia, Oxfam interviewed farmers who were hit by the extreme heat wave in 2010 and then by a less publicized drought in 2012 and found many have got into serious debt.

“In 2010 we were not so bound by loans, we had fewer debts, but now the situation is completely different. We had to take money from the mafia and now that we go to bed, we are afraid they could cut our heads off or the bailiffs could come and take everything from our homes. Today we could basically declare ourselves bankrupt and close down the farm,” one farmer in Russian explained.

There are obviously many interlocking reasons for food insecurity in Pakistan, Central America or Russia (in fact, everywhere) but climatic shocks made worse by global warming have added a potent new threat multiplier and we are only just beginning to unravel how those threats may play out.

In the jargon of food security, impacts on access to food (cutting incomes and spoiling livelihoods, as well as increasing the cost of food) might be more significant than simply affecting food production and simple availability. This is the theme of a new briefing paper by Oxfam, Growing Disruption: climate change, food and the fight against hunger.

The brief says climate change need not–must not–further jeopardize people’s right to food. There is much that must be done. Alongside the resources going into crop research, we need measures that will safeguard people’s livelihoods, food reserves, social protection, and legal frameworks that establish and ensure implementation of the right to food.