Victims are cremated on Sunday after an earthquake devastated Nepal’s heavily crowded Kathmandu valley, killing at least 3,800 and triggering a deadly avalanche on Mount Everest. (Reuters/Navesh Chitrakar)

In 2013, I interviewed Jo Scheuer, a United Nations official advising countries on natural disaster risk reduction. He’d been all over the world, surveying places vulnerable to floods, earthquakes and droughts, and I asked him what natural disaster scenario he feared the most. Without hesitation he replied: “The one that personally keeps me up at night, the one I fear the most is an earthquake in Kathmandu Valley. It’s one of the catastrophic hotspots in the world. We are waiting for it to happen, basically.”

And on Saturday in Nepal, it did. The exact death toll and extent of the damage is still being assessed. But signs point to a major catastrophe, and in crises like this one, the glaring shortage of aid resources will likely translate to more misery on the ground.

[Read full story: Death toll rises to 3,800 in Nepal earthquake]

For the U.N. agencies and nongovernmental organizations specializing in disaster response, this earthquake could not have come at a worse time. The international humanitarian response system — the constellation of groups such as the World Food Program, UNICEF, the Red Cross, Save the Children, Médecins Sans Frontières and others — has never been stretched as thin as they are today. Between crises in South Sudan, the Central African Republic, Iraq, Yemen, the Ebola outbreak in West Africa and Syria, there have never been as many huge humanitarian crises happening all at the same time. In U.N.-speak these are each called “Level 3” emergencies. There is no Level 4.

The unprecedented number of concurrent massive emergencies is undermining these agencies’ ability to adequately respond to any one of these crises — but not for lack of effort or ingenuity. Rather, the capacity of NGOs and U.N. agencies to respond is contingent on the generosity of donors. And right now, donors are not contributing on a scale anywhere near commensurate to worldwide needs.

When man-made or natural disasters strike, relief agencies have to go to donors hat-in-hand to fund their operations. Traditionally, the biggest donors have been the governments of wealthier countries around the world, chief among them the United States, Japan and Western Europe. They put up the money that allows relief organizations to buy and distribute the food, medicine, shelter and other basic necessities of human existence and dignity.

The U.N.’s Office for the Coordination of Humanitarian Affairs typically consolidates these needs into a single appeal, against which donors can contribute to any or all parts of it.

The day the earthquake struck Nepal, there were 15 of these appeals ongoing, totaling $18.67 billion. Donors had so far contributed $3.27 billion, leaving a humanitarian funding gap of an astounding $15 billion. In other words, the international community is paying for less than 20 percent of what relief agencies need to adequately reach everyone in need.

The gaps for some of the major individual appeals are even more glaring. The largest single appeal is for Syrian refugees. Humanitarian agencies need $4.5 billion this year to care for the basic needs of an estimated four million Syrian refugees, as well as the people in their host communities. They have so far received only $791 million, or 17 percent of that appeal. The gap is nearly as bad for people affected by conflict in South Sudan: Only 26 percent of a $1 billion appeal has been funded. And it’s even worse for people in the Central African Republic: An appeal to care for the affected population there is only 14 percent filled. Indeed, in recent years it’s been nearly unheard of for an appeal to reach even 50 percent of its goal.

It’s not that donations have suddenly dried up. It’s that current levels of funding simply aren’t enough, and donors either can’t — or won’t — keep up.

It means less food, shelter, medicine and other bare essentials for populations affected by a crisis. Things got so bad in December that for lack of $64 million, the WFP was forced to cut rations for 1.7 million Syrian refugees. When no traditional donor governments stepped up to fill that gap, the WFP resorted to crowdfunding through social media.

Given its remote location and mountainous geography, the humanitarian response to the Nepal earthquake is going to be costly. Still, professional humanitarians are going to pour into Kathmandu to build shelters, distribute rations and set up emergency hospitals. That is what the world expects of them. What they expect in return is the financial support they require to reach everyone in need. Unfortunately, if present trends continue they will only be able to serve a fraction of the actual needs on the ground.

Our humanitarian system is stretched beyond capacity right now. Unless donors step up in a big way, this earthquake will only exacerbate the problems faced by relief organizations around the world, and they’ll be further stretched when the next natural disaster strikes.