Over the past three years, the production of corn—which is largely grown in the north of the country—has fallen by 1.6 million metric tons, due to "unfavorable climactic conditions" and Boko Haram violence, according to the US Department of Agriculture. Yields of wheat, which is also mostly grown in the north, fell by 15,000 metric tons over the same time period.

The depleted northern soil has pushed both farmers and herders down south. According to Husaini, migration southward by Fulani nomads has increased roughly 50 percent in his region over the past few years. But, as the leader of one nearby farming community put it, "the land is not expanding." Last year, competition for land led to the first clash between farmers and herders that Husaini's community has ever seen. It lasted a month.

That was when 10 of Yakubu Mama's uncles were slaughtered by farmers from the Eggon tribe. Mama, a 42 year-old Fulani herdsman, says through an interpreter that the Eggon militia knifed his relatives to death, "one by one." And since the victims' families were too afraid to go and collect the corpses, the bodies were eaten, Mama says, "by pigs and dogs."

Ten members of Yakubu Mama's family were knifed to death last year. Erika Eichelberger

Husaini says about 200 Fulani herdsmen were killed along with Mama's uncles during the January 2013 violence. There were heavy casualties on the Eggon side, too. Fighting between farmers and herders—which can also take on religious, ethnic, or political veneers—has killed a total of about 8,000 people since 2005, according to the International Crisis Group.

The farmer-herder conflict and the Boko Haram violence in the north are largely separate conflicts. But the two wars do intersect sometimes. After Boko Haram bombed churches and a marketplace in the city of Jos in December 2010, for example, the group said that the attack was revenge for recent Christian violence against Muslim Fulanis.

"Much of the conflict...between Christians and Muslims is about land and access to water," says Vanda Felbab-Brown, an expert on insurgency at the Brookings Institution, "but Boko Haram is tapping to those sentiments and inflaming those sentiments."

As Nigeria's farmer-herder war heats up and more "young people are pushed to the wall," there's a greater chance that young Muslim Fulanis will be sucked into the growing Boko Haram insurgency, says Oluwakemi Okenyodo, the executive director of CLEEN Foundation, a Nigerian security-focused nonprofit.

Since its emergence in 2009, Boko Haram has slaughtered some 5,000 people. The terror organization, whose name loosely means "Western education is a sin," staffs its ranks partly through a mosque and Islamic school set up in 2002 by Mohammed Yusuf, the group's founder. But, as is the case with other insurgent movements around the world, economic hardship also helps drive recruitment. Poverty and unemployment in the north have reinforced the Boko Haram narrative that says the government has been corrupted by Western values, and thus cares more about enriching itself than helping Nigerians, according to a 2011 report by the congressionally-funded United States Institute For Peace.

The Nigerian government offers virtually no safety net to fishermen or cow herders or farmers who can no longer live on barren land. "Crop failure means starvation," says Kwabena Gyimah-Brempong, a research fellow at the International Food Policy Research Institute in Abuja. More and more Nigerians are starving these days. In 2010, 61 percent of Nigerians lived in absolute poverty, defined as barely getting by on minimum levels of food, shelter, clothing. That's up from 55 percent in 2004, according to the most recent poverty survey by the Nigerian statistics agency. It's worse in northeastern Boko Haram country, where about 70 percent live in absolute poverty.

It has become "easy" for Boko Haram to recruit northern Nigerians "because people are already aggrieved and hopeless."

By some accounts, those who sign up for terrorism are motivated as much by economics as by the group's Islamist rhetoric. In a previously unpublished interview with Human Rights Watch, the leader of the Civilian Joint Task Force, the anti-Boko Haram vigilante group made up partly of former Boko Haram members, said that Boko Haram recruitment "has nothing to do with religion, but a lot to do with economic resources." The terror group "swoop[s] on those who don't have any resources whatsoever," says Mausi Segun, an HRW researcher based in northern Nigeria.

Which is why the Nigerian government has begun calling attention to links between environmental degradation, poverty, and violence. Labaran Maku, Nigeria's Minister of Information, told Mother Jones that poverty due partly to crop failures in the north creates "a conducive environment for [terrorism] to prosper." Eziuche Ubani, who chairs the committee on climate change in Nigeria's House of Representatives, says that it has become "easy" for Boko Haram to recruit northern Nigerians "because people are already aggrieved and hopeless and depressed because they are losing their source of livelihood."

The Nigerian military zeroes in on areas hit by agricultural collapse in its anti-Boko Haram raids. The security forces target fish markets in the northern city of Maiduguri, and Segun believes this is because Boko Haram recruits members from the impoverished fishing settlements around the disappearing Lake Chad.

Some Nigerian officials blame global warming for the environmental ravages stoking  the north of the country. Last year, Nigeria's national security adviser, Col. Sambo Dasuki, warned that climate change and desertification are tanking the economy in the northeast, and helping feed the terrorist insurgency there. Sen. Bukola Saraki, the chair of Nigeria's senate committee on the environment, argues much the same.

Nigeria experts caution that while environmental destruction certainly contributes to poverty and violence in the north, there's not enough hard evidence yet to implicate human-caused climate change in the bulk of the ecological disaster. Paul Lubeck, the associate director of African studies at Johns Hopkins School of Advanced International Studies, says the Nigerian government may be overemphasizing the current effects of climate change on the Boko Haram insurgency to deflect blame for its own inability to stem attacks. "The north is faced with a crisis of unimaginable proportions," he says, "and external solutions are very attractive."

But in the coming decades, climate change will begin to take a greater toll across Africa, and violence will likely get worse.

Scientists don't yet know exactly how climate change will affect rainfall in the Sahel—the region could even see more rain. But what is certain is that the area is going to get much warmer. Global temperatures are expected to rise by up to 8.4 degrees Fahrenheit by the end of the century if carbon emissions remain high. In the Sahel, the heat spike will be sharper.

Cattle herders in eastern Nigeria. Erika Eichelberger

Hotter temperatures will likely do serious damage across Africa to crops like wheat, rice, and corn, according to the IPCC, even if climate change ends up bringing heavier rainfall to the region. Wheat yields in sub-Saharan Africa, for example, could fall 35 percent by mid-century. Production of staple crops like sorghum could fall 15 percent by 2050; cassava production could drop by as much as 20 percent, according to the International Food Policy Research Institute.

If such a scenario pans out, "it would be bad," says Jon Padgham, one of the authors of the IPCC report's Africa section, especially "given that food security is already tenuous and population growth projections indicate that Africa will need to produce a lot more food to feed its growing population."

The IPCC says that climate change could intensify resource conflicts and civil wars by aggravating poverty. The Pentagon has warned that it could increase violent extremism around the world. One recent study found that the frequency of violent conflict across the globe could rise by as much as 50 percent by 2050.

Though Nigerian officials have paid lip service to concerns about conflict linked to environmental collapse, terrorism experts say the government is not doing enough to prevent displaced farmers, herders, and fishermen in the northeast from turning to violence—with jobs or a smarter irrigation system or a safety net. The minister of agriculture has several plans to help out farmers, but most have not made it past discussion-group stage. Instead, Nigeria is mostly ignoring the farmer-herder conflict and is fighting Boko Haram with a largely unaccountable military—the government will spend roughly $6 billion this year on security forces whose expenditures are not tracked.

The United States also funds the Nigerian armed forces, to the tune of about $1 million per year, plus $3 million in law enforcement assistance. In May, President Obama sent about 80 US military personnel to Chad to aid in the recovery of the kidnapped Nigerian schoolgirls. But a more effective Boko Haram deterrent may be the millions the US State Department plans to spend next year on anti-poverty and agricultural development initiatives in the country aimed specifically at combating extremism.

Those programs won't be enough, though, if the US and other rich countries don't slam on the brakes on carbon emissions, and if Nigeria doesn't do more for its poor. "If the US and Nigeria don't partner to address climate change in the north, there will likely be repercussions for US interests in northern Nigeria," says Jacob Zenn, an expert on radical groups in northern Nigeria at the Jamestown Foundation. "Possibly effects in the long-term so dire that one day they may be felt on the US home front."

Reporting for this story was funded by a Middlebury College Fellowship in Environmental Journalism.